Intraperitoneal administration of the opiate antagonist, naloxone hydrochloride, resulted in decreased food and water consumption in drug-naive cats. In a cross-over Latin Square design, food consumed by six cats in a one hour period following 23 hours of deprivation, was decreased significantly below control (p less than 0.05) in linear relation to increasing dose (1 mg/kg and 10 mg/kg) of naloxone. Non-linear and time/order effects were not significant. Water consumption was decreased below control in a linear relation to increasing dose (1 and 10 mg/kg) for 5 of 6 cats at the 0.05 significance level. Non-linear and time/order effects on water consumption were not statistically significant for the same 5 cats. These results, and behavioral signs (i.e., vomiting, persistant vocalization, heavy salivation, mydriasis, moderate catatonia, and hissing) occasionally exhibited by four of the six cats in a 1-hr period following injection of the high dose, suggest a malaise-effect of naloxone.