The antiestrogen (ICI 46,474) and antiandrogen (cyproterone acetate) provoke castration-like changes in the pars distalis cytology of adult Rana esculenta. In males, cyproacetate also causes regression of androgen-dependent thumb pads. Effects upon the pars distalis are not direct but rather through the hypothalamus. Stimulatory effects of exogenous estrogen and androgen upon their target organs (both peripheral and central) are selectively inhibited by the respective antisteroidal compound. It seems that these compounds act by competitive action against steroid hormones at the receptor sites in target organs, as has also been suggested in mammals. The antiandrogens, cyproterone and cyproterone acetate, exert masculinizing effects in the course of sex differentiation of gonads of Rana esculenta. These effects are similar to those obtained by androgens.