Experiments described here show that in vivo glucose uptake is impaired in mice given 30 micrograms leptin by intraperitoneal injection 2 hours before an oral glucose tolerance test (GTT). When mice were infused for 7 days with 10 micrograms/day leptin, the 4-fold increase in circulating leptin caused a transient hypophagia, a sustained weight loss and significantly inhibited insulin release in response to an oral GTT. Adipocytes from these mice were not insulin responsive whereas insulin-stimulated muscle and liver glycogen synthesis were increased. In contrast, leptin added to 2 hour in vitro incubations had an insulin-like effect on muscle glucose utilization and augmented insulin stimulation of adipocyte lipid synthesis. Thus, normal mice treated chronically with leptin develop tissue specific changes in insulin sensitivity and compensate for inhibition of glucose-stimulated insulin release. The contrasting response to acute leptin exposure suggests these changes are not a direct effect of the protein.