Immature rats were treated with PMSG followed 56 h later by 10 i.u. hCG. Follicles were removed at intervals after hCG injection. Transient increases in progesterone, testosterone and oestradiol synthesis were first evident 1 h after hCG, but values peaked at 3-5 h and returned to control levels by 10 h. Increased synthesis of PGE-2 and PGF-2 alpha was not evident until 3 h and peaked at more than 10 h after hCG. Ovulation began between 8 and 10 h after hCG and 83% of animals had ovulated within 12 h. Doses of 90 or 1800 micrograms indomethacin given together with hCG substantially inhibited ovulation and PG synthesis, but only the higher dose inhibited the hCG-induced elevation of progesterone and testosterone synthesis; hCG-induced oestradiol synthesis was not affected by either dose of indomethacin. We conclude that the peak of PG synthesis after hCG treatment related closely to the timing of ovulation; the steroidogenic response to hCG was not blocked by doses of indomethacin sufficient to inhibit synthesis of PGE-2 and PGF-2 alpha by more than 80%.