Two groups of rats were treated with seven daily injections of either saline or d-amphetamine (3 mg/kg IP). On the 2 days following the last injection, rats were tested according to a counterbalanced experimental design, each animal receiving, immediately prior to the beginning of the dark phase, saline on one day and the highly selective kappa-opioid agonist trans-+/-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)cyclohexyl]-benzene-acetamide methanesulfonate hydrate [U-50,488H (U50)] on the other. A microcomputer-controlled data acquisition system was used for the structural analysis of the feeding and drinking responses to amphetamine and U50. U50 enhanced feeding and depressed drinking in the first hour. The increased food intake was probably the result of the effect of U50 on the development of satiation and duration of satiety. Chronic amphetamine potentiated the prophagic effect but not the antidipsogenic effect of U50. The structural analysis demonstrated that the characteristics of the prophagic effect of U50 were amplified but not changed.