Seasonal plasma and testicular estradiol levels were measured in the male frogs, Rana esculenta, by radioimmunoassay. In plasma samples a simultaneous measurement of androgens was carried out in order to investigate a possible relationship between androgens and estradiol-17 beta. Concomitantly with the estradiol-17 beta peak in plasma and testes during the April-May period, plasma androgens sharply decreased.