Weanling genetically obese (C57BL/6J-ob/ob) and lean (+/?) mice were given access to either a standard laboratory diet or the standard diet plus a 32% sucrose solution. At the end of a 4-week period, animals were sacrificed and opiate receptor binding determined. Both obese and lean mice given access to sucrose consumed approximately 30% more calories per day than animals given access to the standard diet alone. Obese animals consumed significantly more calories from the sucrose solution than the lean animals. Genetically obese animals weighed more than lean littermates throughout the course of the study. Differences in body weight due to sucrose supplementation in both genetically obese and lean mice were significant by day 10 and increased in magnitude until the termination of the study. Whereas there were no significant differences in specific opiate receptor binding (pmol 3H-naloxone bound/mg brain protein) between the genetically obese and lean animals, opiate receptor binding was significantly greater in genetically obese animals given access to sucrose than in obese animals which had access only to the standard diet. These data demonstrate that the sucrose-induced model of obesity functions in mice and that giving ob/ob mice access to sucrose in addition to a standard laboratory diet results in increased opiate receptor binding.