We investigated the influence of a specific serotonin receptor blockade on pituitary hormone secretion in normal cycling women and postmenopausal women not receiving hormone replacement therapy. Serotonin receptor blockade was performed by using the HT-3 receptor subtype antagonist ondansetron as an i.v. bolus injection of 8 mg. Blood samples were taken before as well as 20, 30 and 40 min after ondansetron administration for the estimation of LH, FSH, and TSH. We could not find any hormonal changes in the normal cycling women. The postmenopausal women showed a significant decline in LH secretion, whereas FSH and TSH levels remained unchanged. Our results suggest serotoninergic involvement in LH secretion in postmenopausal hypoestrogenic hypergonadism.