The present study has examined the effect of a single in vivo intraperitoneal injection of the adipocyte-derived hormone, leptin, on the in vitro lipolysis of fat cells of different types of mice. Administration of 1 and 10 mg leptin per kg body weight to ob/ob mice significantly increased (P < 0.0001) the basal lipolytic activity compared to ob/ob mice receiving vehicle solution (phosphate-buffered saline, PBS). The highest leptin dose tested (10 mg/kg body weight) produced a threefold increase in basal lipolysis. In lean mice administration of 10 mg leptin per kilogram of body weight produced an increase in basal lipolysis of 52.7% (P < 0.01). However, in db/db mice none of the three leptin doses injected had a significant effect on the lipolytic activity of adipocytes relative to basal lipolysis observed in db/db mice injected with PBS only. These data provide evidence for a lipolytic effect of leptin on white adipose tissue, which operates independently from changes in food intake, body weight, and the size of the fat stores.