Atlantic salmon, Salmo salar, and several species of whitefish, Coregonus nasus, C. peled, C. Lavaretusbaeri, males were treated with a long-acting medical androgen preparation - "Testoenatum". This preparation contains 24 mg of testosterone enanthate and 111 mg of testosterone propionate in 1 ml peach oil solution. Multiple (2–5) or single intraperitoneal injections of a standard dose (0.1 ml/kg fish body weight) were used. Treatment performed shortly before the beginning or within the breeding season resulted in a significant stimulation of spermiation, improvement of sperm parameters and extention of the gonad functional maturity period for 1–4 months. Low water temperatures (0.5–l.0 °C) seem to be essential to obtain a long-lasting effect after one injection. The histology of the pituitary gland suggests that the gonadotropic cells were activated through a positive feedback mechanism. Testoenatum suppressed Sertoli cells' phagocytary activity.