SUMMARY: Immature mice were induced to ovulate by a single injection of pregnant mare serum gonadotrophin (PMS). Additional treatment with 5-hydroxytryptamine (5-HT) during the final 24 hr. of the experiment caused significant alteration in the dose-response relationship for PMS, the ovulatory response to low doses of PMS being increased and that to high doses reduced. Both lysergic acid diethylamide and methysergide caused a consistent and significant reduction in the incidence of ovulation induced under the same test conditions. Neither 5-HT nor its antagonists influenced spontaneous ovulation in adult mice of a different strain under the conditions tested. Therefore, if 5-HT is involved at some stage in the processes leading up to ovulation when this is induced in immature mice, either it does not have the same importance or its effect is less easily modified under the physiological conditions of spontaneous ovulation in mice of the strain used.