Implants of progesterone on the day of dioestrus II in the hamster induced on the following day an increase in circulating levels of progesterone (6\m=.\0\ m=+-\0\m=.\7ng/ml, N = 8; sesame oil controls, <0.5 ng/ml, N = 6) and a decline in serum levels of LH (5\m=.\3\ m=+-\ 0\m=.\4 ng/ml; controls 12 \ m=+-\ 2 ng/ml) and oestradiol (10 \ m=+-\ 2 pg/ml ; controls 69 \ m=+-\ 5 pg/ml). The production of androstenedione and oestradiol by antral follicles in vitro was reduced in progesterone-treated

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... amsters when compared with controls, but progesterone production was not affected. Aromatizing activities of antral follicles were the same in progesterone-treated and sesame oil-treated hamsters. Androstenedione production by theca was significantly less in progesterone-treated hamsters than in controls. On dioestrus II, LH replacement therapy (200 \ g=m\ g ovine LH by osmotic minipump inserted s.c.) prevented the decline in follicular androstenedione and oestradiol production induced by progesterone alone, and also prevented the decline in thecal androstenedione production in vitro. The results indicate that exogenous progesterone on dioestrus II lowers circulating levels of LH by the following day, inhibits thecal androstenedione production and thus reduces follicular oestradiol production without alteration in aromatizing ability.