To address the issue of feline manic responses to morphine, studies were designed to examine the effects of low doses of intravenous morphine on cat behavior with emphasis placed on motor activity changes. An adaptation of an existing scoring system was used in an effort to quantify and describe behavioral patterns or shifts in activity patterns occurring during a cycle of addiction. The acute administration of 1, 2 or 4 mg/kg of morphine induces a response pattern characterized by sitting with fixed staring. An increase in motor activity with dose was also observed. When naloxone was administered prior to morphine injection all behavioral changes normally elicited by morphine were blocked completely. After 7 days of daily morphine injections certain changes in activity profiles occurred, indicating that some degree of tolerance although not complete, was beginning to occur by this time. Naloxone administered to cats maintained for 12 days on1, 2 or 4 mg/kg/day consistently produced a number of withdrawal signs including wet-dog shakes and a catatonic-like posturing. This is the first report describing behavioral responses of the cat during a complete cycle of addiction to low doses of morphine. We not only found that morphine will elicit definite quantifiable changes in behavior in the absence of any feline mania but that cats become readily dependent on these low doses of morphine as evidenced by a characteristic naloxone precipitated withdrawal syndrome.