Obese and lean Zucker rats received bilateral electrolytic lesions of the lateral hypothalamus (LH) at 10 wk of age; control obese and lean rats were sham lesioned. After lesioning the body weights of both obese and lean animals were first reduced and then maintained until being killed (32 wk) at a stable percentage of the nonlesioned control levels (74.5 and 78.3%, respectively). Carcass analysis revealed that the adipose tissue mass was significantly lowered by LH lesions in both the obese and lean animals. Percent carcass fat of lesioned lean rats was less than that of controls (15.0 vs. 23.5%) due to the presence of slightly, but not significantly, smaller and fewer adipocytes. Though absolute levels of fat were likewise lowered in obese rats, their percent carcass fat remained at control levels (52.0 vs. 53.0%) due to equivalent decreases in other body compartments. In the lesioned rats the reduced levels of adipose tissue were associated with a significant reduction in adipocyte number; adipocyte size was unchanged. It is concluded that the lateral hypothalamus of obese Zucker rats participates in the regulation of body weight in the same manner as in lean rats. The differences noted in percent carcass fat between LH lesioned lean and obese Zucker rats are apparently related to the obese animal's known propensity to sequester energy in the form of lipid.