SUMMARY: The oestrous cycle of the hamster is lengthened by 2 to 3 days when 2·5 or 5·0 mg progesterone are injected on day 1 of the cycle (day of ovulation). Antral follicles develop on day 2 but their growth is significantly retarded in progesterone-treated hamsters. Administration of progesterone is followed within 6 h by an abrupt decline in serum FSH and LH concentration but by day 3 the level of FSH is higher than normal. Serum LH is slower to recover in hamsters receiving 5 mg progesterone but normal levels are also restored by days 3 or 4. Increased serum levels of progesterone are maintained until days 3–5 in animals injected with progesterone. Despite the presence of antral follicles on day 2 and the fairly prompt restoration of normal levels of gonadotrophins, the serum concentration of oestradiol is suppressed for several days in the progesterone-treated hamster. This suggests that progesterone, in addition to affecting the hypothalamic-pituitary system, may also directly inhibit oestrogen secretion by the antral follicles.