Testicular serotonin (5HT) concentrations were determined by HPLC in the testes of rats treated neonatally with oestradiol benzoate (EB) and in adult rats treated with the Leydig cell cytotoxic ethylene dimethane sulphonate (EDS). 5HT concentrations were related to mast cell numbers. EB-treated rats showed an accumulation of mast cells in the testes at 35 and 70 days of age and increased 5HT concentrations in both the interstitial fluid and the testicular capsule, whereas no increases in 5HT concentrations or in the number of mast cells were found for the ventral prostate of these animals. On the contrary, 5HT concentrations were not related to the number of Leydig cells. In EB-treated rats, in which Leydig cells were nearly absent at 35 days of age, 5HT concentrations were significantly increased. Furthermore, EDS-treated rats did not show significant changes in 5HT concentrations, in spite of the elimination of Leydig cells. These data suggest that mast cells are a major source of serotonin in the rat testis.