The aim of the present study was to assess the effect of chronic naltrexone treatment on daily patterns of food intake in food-deprived and free-feeding rats. In experiment 1, Wistar male rats had continuous access to food and water, while in experiment 2 they were deprived of food for 12h/day. Animals in both experiments were studied as follows: a baseline period (7days), followed by a treatment period (14days) with either saline or naltrexone at 10mg/kg/day. Finally, a post-treatment period (7days) was assessed. Food and water consumption were measured every 2h after the naltrexone or saline injection for 12h and once more 12h later. Experiment 1: Food intake was higher in the naltrexone group 10h after injection. Total food intake and body weight gain were higher in the naltrexone group than in the saline group in the second week of treatment and in the post-treatment period. Experiment 2: The overeating observed in the saline group in the hours following the 12h of the food deprivation period was suppressed by naltrexone, though total daily food intake was not affected. Body weight gain was initially reduced by naltrexone, but a rebound effect was observed during the post-treatment period in the naltrexone group. Naltrexone produced a differential effect on food intake and body weight that depended on the rats' food deprivation status. These results could be explained in terms of opioid receptor up-regulation that enhances the rewarding effects of food or by naltrexone-produced changes in palatability.