SUMMARY Single injections of melatonin, 5-hydroxytryptamine or pineal powder on the day of operation significantly blocked compensatory ovarian hypertrophy in unilaterally ovariectomized mice in a dose-related fashion. Inhibition of compensatory ovarian hypertrophy by melatonin was most effective when the compound was given on the day of dioestrus and least effective on the day of pro-oestrus. These results support the conclusion that the pineal gland elaborates one or more substances which have an antigonadotrophic action in the female rodent.