Inhibition of ovulation by indomethacin in rats

Indomethacin, a specific prostaglandin synthetase inhibitor, was found to block ovulation in rats when scheduled at 1200h on day 4 of the cycle. The same regimen was ineffective at 1800h, but its antiovulatory efficacy was estimated to be about 50% when given at 1600h. An injection of indomethacin given concurrently with prostaglandin F2a allowed ovulation in 80% of the test animals. None of the animals, irrespective of the drug schedule, became pregnant on mating with the fertile males but showed a remarkable decidual cell reaction (DCR) in response to unilateral uterine traumatization. The anti-ovulatory or intinidation effect of indomethacin is discussed.