Effects of serotonin on ovulation have been studied in immature female rats treated with pregnant mares'' serum gonadotropin (PMS). A smaller number of ova were counted in the oviducts of rats that were given either 1, 10 or 25 mg/kg body weight of serotonin than in control animals. Several rats in each dosage group did not ovulate, but all of the control animals ovulated. Other groups of immature female rats, beginning at age 24 days, were injected daily with serotonin until the time of the opening of the vagina. The vaginal opening time was delayed and the ovaries weighed significantly less than in the control rats. The results indicate that serotonin not only suppresses luteinization, as reported previously, but also affects ovulation and vaginal opening time in immature rats.