Summary. Immature Wistar rats were induced to ovulate by treatment with PMSG and hCG. Control animals ovulated 43·5 ± 0·36 ova/rat. Intraperitoneal injection of rotenone doses of 0·125, 0·25 and 0·50 mg/kg reduced the ovulation rate to 24·0 ± 3·08, 8·0 ± 0·88 and 1·5 ± 0·44 ova/rat, respectively. The rotenone significantly reduced ovarian cytochrome oxidase activity and progesterone production, but not production of oestradiol or testosterone. Thyroxine treatment at a dose of 5 mg/kg s.c. reversed the rotenone inhibition of ovulation. The results suggest that an increase in mitochondrial respiration is an essential feature of the ovulation process in mammals.