The twofold question has been examined whether the regression of the Müllerian ducts in the chick embryo is determined by testosterone and whether this androgen is being secreted by the testes. Müllerian ducts from 8-day-old male chick embryos were immersed in a 1 mu 1 volume of testosterone solutions ranging from 0,1 mM-nM. Regression, which had already begun at the time of the explantation in vitro, did not continue. After culturing testes from 10 day old chick embryos in the presence of 14C labelled sodium acetate, progesterone or dehydroepiandrosterone, search was made for the following radioactive androgens in the media: testosterone, androstenedione, androsterone, dehydroepiandrosterone, androstanediol and androstenediol, but none of them could be identified by recrystallization to constant specific activity. Filters were used just to get some idea about the molecular weight of the testicular hormone responsible for the regression of the Müllerian ducts. It seems that it would lie around 50 000. In view of these results, it is difficult to maintain the often advanced hypothesis which considers testosterone as the active substance. This may be a protein.