Opioid antagonists have been reported to reduce water and food intake. In the experiments, ingestive behavior was sometimes induced using stressful stimuli. These experiments were conducted to determine the effects of chronic administration of an opioid antagonist on appetitive behaviors of minimally stressed rats. Twelve 4-week old rats were randomly separated into two equal-sized groups, anesthetized with ether, and implanted subcutaneously with poly(epsilon-caprolactone) capsules. One group received naltrexone in the capsules, the other group received vehicle. These capsules released the drug at a rate of approximately 250 microgram/day for over 8 weeks. Gross metabolic behavior (food intake, water intake, urine output, fecal output, and body weight) was monitored for 8 weeks. At three times during the 8 weeks, food deprivation (24 hr)-induced feeding was tested over a period of 60 min. At the end of the eighth week, the animals were subjected to an analgesic test in order to confirm the efficacy of the naltrexone dose. No differences in food intake, body weight, fecal output, or deprivation-induced feeding were observed between the control and naltrexone groups. Water intake and fluid output of the naltrexone group were significantly reduced compared to the control group. These results suggest that endogenous opioids may mediate mechanisms of tonic drinking behavior and cannot be explained as representing effects on stress-induced ingestion.