Previous research has suggested that chronic intake of palatable foods and fluids enhances the activity of the endogenous opioid system. To examine this suggestion, the effect of naltrexone on food intake was examined in male Long-Evans rats with or without prior exposure to palatable solutions. In Experiment 1, rats were fed laboratory chow alone or laboratory chow and a 32% sucrose solution, and in Experiment 2, were fed chow alone, chow and a 32% Polycose solution, or chow and a 0.15% saccharin solution for three weeks. The sucrose, Polycose, and saccharin solutions were removed 18 h prior to drug administration. Rats then received injections of naltrexone hydrochloride (0.0, 0.3 or 3.0 mg/kg. sc) and chow intakes were measured during the subsequent 1, 2, 4, 6 and 24 h. Naltrexone injections had minimal effects on intakes of animals which previously had consumed only chow. In contrast, naltrexone led to significant dose-related decreases in chow intakes in rats which had previously consumed the sucrose, Polycose, or saccharin solutions. These results provide confirmation for the suggestion that chronic intake of palatable solutions alters the activity of the endogenous opioid system.