The effects of estradiol on the secretion and turnover of serotonin in the hypothalamic fragments of male tilapia, Oreochromis mossambicus, were studied using a static incubation system. The quantitative analysis of serotonin and its related metabolite, 5-hydroxyindoleacetic acid, were performed by high-performance liquid chromatography with electrochemical detection. The hypothalamic fragments were incubated with 17 beta-estradiol at a concentration of 2 x 10(-8), 8 x 10(-8), 2 x 10(-7), 4 x 10(-7), or 4 x 10(-6) g/ml. The low dose of estradiol, 2 x 10(-8) g/ml, had no effect on the concentration of serotonin and 5-hydroxyindoleacetic acid or serotonin turnover in the hypothalamic incubation media. The moderate doses of estradiol 8 x 10(-8) and 2 x 10(-7) g/ml, increased the concentrations of serotonin and 5-hydroxyindoleacetic acid in the hypothalamic incubation media, but had no effect on the serotonin turnover. The high doses of estradiol, 4 x 10(-7) and 4 x 10(-6) g/ml, did not alter the serotonin concentration in the hypothalamic incubation media, but increased the 5-hydroxyindoleacetic acid concentration and serotonin turnover. These results demonstrate that the moderate dose of estradiol increases the serotonin activity by increasing the serotonin concentration, whereas the high dose of estradiol increases the serotonin activity by increasing the ratio of 5-hydroxyindoleacetic acid and serotonin. However, the serotonin concentration is homeostatically maintained in the extracellular fluid of hypothalamus under the high dose of E2 treatment.