http://www.fanciers.com/cat-faqs/behavior.shtml

It is possible for cats to stop using the litter box or to have trouble
learning in the first place.
Do NOT ever try to discourage a cat's mistakes by rubbing its nose in it. It
never worked for dogs and most certainly will not work for cats. In fact,
you wind up reminding the cat of where a good place to eliminate is!

Potential CAUSES for failure to use litterbox:

a.. MEDICAL PROBLEMS:
a.. diarrhea (many causes)
a.. small intestinal- soft to watery
b.. colitis (inflamed colon)- mucus in stool, blood, straining
b.. urinary bladder inflammation
a.. FUS
b.. Bacterial infections
c.. trauma
d.. calculi (bladder stones)
e.. tumors
c.. polydipsia/polyuria
(excessive water volume consumed and urine voided: upper water intake
for cats is 1oz/lb; most cats drink considerably less than this)
a.. diabetes insipidus
b.. diabetes mellitus
c.. kidney disease
d.. liver disease
e.. adrenal gland disease
f.. pyometra (pus in the uterus)
g.. hypercalcemia (high blood calcium)
h.. others
b.. TERRITORIAL MARKING
a.. intact female in heat
b.. intact male spraying
c.. marking of peripheral walls particularly near windows may be from
presence of outdoor cats
d.. may be triggered by over-crowding of indoor cats
e.. previously neutered cat has a bit of testicular or ovarian tissue
remaining, possibly resulting in a low level of hormone which could trigger
marking
f.. neutered male with sexual experience exposed to female in heat
c.. LITTER BOX PROBLEMS
a.. overcrowding: too many cats using same box
b.. failure to change littter frequently enough -- some cats won't use a
dirty box
c.. failure to provide constant access to litterbox
d.. change in type of litter used
e.. change in location of litterbox
f.. unfamiliar, frightening, or loud objects near box: dishwasher, etc.
g.. food and water too close to litterbox
h.. objectionable chemical used to wash or disinfect litterbox
i.. location preference: your cat may want the box in a different
location
j.. texture preference: your cat doesn't like the feel of the litter
k.. failure to cover litter: learned process from parents
a.. use of litterbox is instinctive
b.. cats that don't cover litter may be more prone to litterbox
problems
c.. your cat may be indicating texture preference problem
d.. PSYCHOLOGICAL STRESS
(most common manifestation is inappropriate urination)
a.. addition or subtraction of other pets in household
b.. visitors, company, parties, redecorating, construction, or any type
of commotion
c.. a move to a new environment
d.. change in routine or schedule: a new job or working hours
e.. their return from boarding or hospitilization
f.. interaction problem with other pets or cats
a.. cats are asocial rather than antisocial; in the wild each has a
territory and period of contact with others in the group (and only one male
per group)
b.. a closed environment will create a greater degree of interaction
than some cats prefer. The more cats in a household, the greater the degree
of interaction
e.. CHEMICAL ATTRACTION OF PREVIOUS "ACCIDENTS:"
a.. likely to produce repeated visitations to the same spot
b.. may induce urination by other members of a multi-cat household
c.. you may have moved to a residence previously occupied with other
dogs and/or cats
Treatment
a.. Rule out medical problems FIRST
a.. complete history and physical
b.. stool/GI workup for diarrhea (if needed)
c.. urinalysis for inappropriate urination to rule out an infection
d.. workup for polydipsia/polyuria
e.. important to check all cats of a multi-cat household
a.. last cat seen misbehaving may be responding to chemical attraction
and not be an instigator
b.. more than one cat could have problem
f.. treat/correct medical problems first. Behavioral problems can only
be diagnosed in a healthy cat
b.. Territorial marking
a.. neuter all cats (check history of neutered cats; retained testicle
in male or signs of heat in female)
b.. prevent other cats from coming around outside of house, close
windows, blinds, and doors
c.. prevent overcrowding in multi-cat households
c.. Litter box problems
a.. provide a box for each cat
b.. change litter daily
c.. provide constant access to a box
d.. go back to previously used brand of litter and/or discontinue new
disinfectant
e.. move box to where it was previously used
f.. eliminate new or frightening noise near litterbox
g.. move food and water away from litterbox
h.. if cat is only going in one spot, put the litterbox at the exact
location and gradually move it back to where you want it at the rate of one
foot per day
i.. if there are several places, try putting dishes of cat food in those
areas to discourage further elimination there
j.. experiment with different textures of litter (cats prefer sandy
litter)
k.. use a covered litterbox for cats that stand in box but eliminate
outside of it
d.. Psychological Stress
a.. eliminate if possible
b.. try to provide each cat at home with its own "space"
a.. use favorite resting areas to determine
b.. provide separate litterboxes near each space if possible
c.. cubicles, boxes, shelves, crates are effective for this
c.. tranquilizers sometimes work well in multicat situations
e.. Chemical attraction
a.. dispose of all soiled fabric or throw rugs if possible
b.. 50% vinegar or commercial products may be used
c.. steam cleaning may help
d.. repellants may help
e.. do not replace carpeting until problem entirely solved or it may
start all over again on your new carpeting
f.. Confinement
(In portable kennel with litterbox, (with appropriate corrections) to stop
further inappropriate behavior while medical and/or other problems are being
treated.)
a.. particularly beneficial for transient stress induced problem
b.. may allow acclimation to stress situation where source of stress
cannot be eliminated
c.. procedure
a.. choose an area that can be a permanent location of litterbox
b.. keep cat confined to this area 4-6 weeks when not under your
direct visual supervision (if your cat attempts elimination outside of
kennel when you are watching, squirt with water pistol as soon as
elimination posture is attempted and put cat back in kennel)
c.. if cat is using box regularly for 4 to 6 weeks when not under your
gradually give access to larger and larger areas of your home, one room or
hallway at a time
a.. allow 1 week of good behavior in the new area before adding the
new room
b.. never increase access area until you are 100% certain cat's use
of litterbox is 100%
c.. if accident occurs, re-evaluate this material to make sure
litterbox problem or something else didn't trigger
d.. begin confinement over again and double intervals for relapses
g.. For inappropriate urination problems in which all else fails and the
alternative is euthanasia, hormone therapy may be attempted.
a.. only for neutered cats only 50% effective
b.. side effects may include increased appetite (common), depression or
lethargy (less common). Long term use might have side effects such as:
mammary enlargement, adrenocrotical suppression, and diabetes mellitus.
c.. usually requires lifelong maintenance on regular intermittent basis
d.. very dangerous drug; use borders malpractice -- should be reserved
for cats who will be put to sleep if problem is not solved
a.. immunosuppressive
b.. weight gains predisposing to obesity
c.. mammary gland development
d.. feminization of males
e.. may induce latent diabetes
e.. dosage is initiated daily for 7 day trial; if effective, then dosage
is tapered to least effective amount given every other day every one to two
weeks
f.. relapses may be expected when drug is discontinued