Gestamimetic amounts of progesterone enhance basal and glucose-stimulated insulin production. Contraceptive doses of synthetic progestins cause a moderate increase or no change in glucose-stimulated insulin production, depending on route of administration and species tested. Estrogens potentiate the insulinotropic effects of progesterone and the synthetic progestins. Basal serum triglyceride concentrations are generally unaffected by progesterone or 17 alpha-acetoxyprogesterone treatment but may decrease during 19-nortestosterone administration. Glucose tolerance does not change during treatment with gestamimetic doses of progesterone alone but may improve in rats and monkeys during concurrent estrogen administration. By contrast, deterioration of glucose tolerance is observed in women treated concurrently with synthetic estrogen plus 19-nortestosterone derivatives and, occasionally, with 19-nortestosterone derivatives alone. No consistent changes in glucose metabolism have been observed after treatment with 17 alpha-acetoxyprogesterone derivatives alone. The cause of the species-related differences in glucose metabolism during 19-nortestosterone treatment is obscure.