OBJECTIVE: Mammalian target of rapamycin (mTOR) and its downstream target S6 kinase 1 (S6K1) mediate nutrient-induced insulin resistance by downregulating insulin receptor substrate proteins with subsequent reduced Akt phosphorylation. Therefore, mTOR/S6K1 inhibition could become a therapeutic strategy in insulin-resistant states, including type 2 diabetes. We tested this hypothesis in the Psammomys obesus (P. obesus) model of nutrition-dependent type 2 diabetes, using the mTOR inhibitor rapamycin. RESEARCH DESIGN AND METHODS: Normoglycemic and diabetic P. obesus were treated with 0.2 mg x kg(-1) x day(-1) i.p. rapamycin or vehicle, and the effects on insulin signaling in muscle, liver and islets, and on different metabolic parameters were analyzed. RESULTS: Unexpectedly, rapamycin worsened hyperglycemia in diabetic P. obesus without affecting glycemia in normoglycemic controls. There was a 10-fold increase of serum insulin in diabetic P. obesus compared with controls; rapamycin completely abolished this increase. This was accompanied by weight loss and a robust increase of serum lipids and ketone bodies. Rapamycin decreased muscle insulin sensitivity paralleled by increased glycogen synthase kinase 3beta activity. In diabetic animals, rapamycin reduced beta-cell mass by 50% through increased apoptosis. Rapamycin increased the stress-responsive c-Jun NH(2)-terminal kinase pathway in muscle and islets, which could account for its effect on insulin resistance and beta-cell apoptosis. Moreover, glucose-stimulated insulin secretion and biosynthesis were impaired in islets treated with rapamycin. CONCLUSIONS: Rapamycin induces fulminant diabetes by increasing insulin resistance and reducing beta-cell function and mass. These findings emphasize the essential role of mTOR/S6K1 in orchestrating beta-cell adaptation to hyperglycemia in type 2 diabetes. It is likely that treatments based on mTOR inhibition will cause exacerbation of diabetes.