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