Localization and peculiarities of steroid-producing cells of gorbuscha fry gonads were studied in normal conditions and after experimental conditions. In females steroid-producing cells are found also in the follicle tunica and between interstitial cells, greatly differing in ultrastructure in these two localities. In males, these cells are located only in the testicular tunica, being homogeneous in ultrastructure. Chronical hormonal treatment of the fry with oestradiol-dipropionate induced in males, contrary to females, a considerable increase in the number and functional activity of steroid-producing cells and the appearance of these cells in the stroma of testis also. In females only a mass appearance of lipid-like droplets in the cytoplasm of steroid-producing cells is observed, which is also characteristic of males. The revealed morpho-functional peculiarities of steroid-producing cells are discussed in terms of their presumed important role in regulation of sex differentiation in fishes.