Gestational diabetes coincides with elevated circulating progesterone levels. We show that progesterone accelerates the progression of diabetes in female dbdb mice. In contrast, RU486, an antagonist of the progesterone receptor (PR), reduces blood glucose levels in both female WT and dbdb mice. Furthermore, female, but not male, PR-- mice had lower fasting glycemia than PR++ mice and showed higher insulin levels on glucose injection. Pancreatic islets from female PR-- mice were larger and secreted more insulin consequent to an increase in beta-cell mass due to an increase in beta-cell proliferation. These findings demonstrate an important role of progesterone signaling in insulin release and pancreatic function and suggest that it affects the susceptibility to diabetes.