Preovulatory follicles from adult hamsters on the morning of pro-oestrus were used in this study. Serotonin stimulated oestradiol production by preovulatory follicles during a 5-h incubation in 1 ml Krebs-Ringer bicarbonate glucose medium containing isobutylmethylxanthine (0.1 mmol/l; IBMX) and androstenedione (1 mumol/l). The enhanced oestradiol production by serotonin was dependent on the dose of IBMX and androstenedione. Mianserin, a serotonin type-1 and serotonin type-2 receptor antagonists, prevented the serotonin-enhanced oestradiol production in a dose-dependent manner. Ketanserin, a specific serotonin type-2 receptor antagonist, was ineffective in blocking the action of serotonin, indicating that the effect of serotonin was mediated by the serotonin type-1 receptor. In the presence of androstenedione (1 mumol/l), serotonin was unable to enhance oestradiol production in isolated granulosa cells. It was also unable to enhance oestradiol production in early atretic follicles; atresia was induced experimentally by an injection of phenobarbital in order to prevent ovulation. The data indicate that serotonin stimulates oestradiol production by hamster preovulatory follicles in vitro. The mechanism of action of serotonin involves an intact healthy follicle, a serotonin type-1 receptor and possibly cyclic AMP. The increased oestradiol secretion might be related to increased androgen production by the follicle and increased permeability (leakiness) of the follicle to androstenedione which serves as substrate for aromatization to oestradiol by the granulosa cell.