Indomethacin, an inhibitor of cyclooxygenase that generates prostaglandins (PGs) from arachidonic acid, and 2 alpha,4 alpha,7-4,5-epoxy-17-hydroxy-4,17-dimethyl-3-oxoandrostane- 2-carbonitrile (epostane), an inhibitor of 3 beta-hydroxysteroid dehydrogenase that generates progesterone from pregnenolone, are both potent inhibitors of ovulation. This report compares the dose-dependent effects of these two inhibitors on ovarian levels of 5-, 12-, and 15-hydroxyeicosatetraenoic acid methyl ester (HETEs), prostaglandin E2 (PGE), prostaglandin F2 alpha (PGF), progesterone, 17 alpha-hydroxyprogesterone, 17 beta-estradiol, 4-androstene-3,17-dione, and testosterone during ovulation in 25-day-old immature Wistar rats. The ovulatory process was initiated by 10 IU of human chorionic gonadotropin (hCG). Indomethacin was given at 3 h after hCG in doses ranging from 0.0316 to 10.0 mg/rat. A dose of 0.1 mg/rat was the lowest dose to significantly reduce the ovulation rate from the control level of 70.5 +/- 5.8 ova/rat. This dose also reduced 15-HETE, but not 5-HETE, 12-HETE, or the steroids. PGE and PGF were strongly inhibited by an even lower dose of indomethacin (0.0316 mg/rat), but this dose did not affect the ovulation rate. Epostane was given at 3 h after hCG in doses ranging from 0.1 to 5.0 mg/rat. A dose of 1.0 mg/rat was the lowest dose to significantly inhibit ovulation. This dose also reduced the ovarian levels of 15-HETE and progesterone but not 5-HETE, 12-HETE, PGE, PGF, or the other steroids. The results indicate that the ovulation rate is most closely correlated to ovarian 15-HETE levels.