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, Di Matteo, L., Minucci, S., Fasano, S., D'Antonio, M. & Chieffi, G. (1986): Plasma and testicular estradiol and plasma androgen profile in the male frog Rana esculenta during the annual cycle - Gen. Comp. Endocrinol. 64: 401-404. Wijnands, H. E. J. & van Gelder, J.J. (1976): Biometrical and serological evidence for the occurrence of three phenotypes of green frogs (Rana esculenta complex) in the Netherlands - Netherlands J. Zool. 26: 414-424. Authors' addresses: András Gubányi Zoological Department Hungarian Natural History Museum 1088 Budapest, Baross u. 13 Hungary Tibor Fehér 1st Department of Medicine Semmelweis University Medical School Budapest, Hungary György Kéri 1st Institute of Biochemistry Semmelweis University Medical School Budapest, Hungary József Pekli Tropical and Subtropical Department University of Agriculture Gödöllő,' Hungary