In the present study, we have utilized 17beta-estradiol to induce the increase of mast cell number in order to verify the melatonin effect on mast cell accumulation in the frog testicular interstitium. Data obtained from in vivo experiments confirm that 17beta-estradiol increases the mast cell number and indicate a melatonin-inhibitory role in their accumulation in the frog testis. In addition, melatonin interferes with the effects of estradiol on the increase of mast cell number in short-term cultured testes, and this result has also been obtained in a dose-response experiment at physiological concentration. The data suggest that melatonin acts on mast cell number directly via its local action in the frog gonads. In conclusion, our study shows, for the first time, that melatonin may interfere, probably via estrogen receptors, with the differentiation and/or proliferation of mast cells induced by estradiol treatment either in vivo or in vitro in the testis of the frog Rana esculenta.