Mono-sexual trout breeding through the use of steroids is derived from the fact that female trout grow faster than males and have meat of better quality and higher carcass efficiency. Treating male trout with estradiol results a phenotypical change of sex, leading to decreased weight of viscera. Some steroid compounds can lead to different results in body development and composition of various kinds of fish. In this study the effects of oral administration of 20 mg of estradiol valerate per kilogram of administered diet were studied in 35-day-old rainbow trout. The protein, fat, ash and Moisture contents of fish-meat, carcass and fillets percentages and the hepato-and gonadosomatic indexes were measured as well as the amount of residual estradiol valerate or metabolite that might have remained in fish-meat. Compared to control, there were no changes in moisture, protein, fat and ash contents of fish-meat and in the fillet percentage of the trout. The mass percentage of viscera and the gonado-and hepatosomatic indexes decreased while the carcass percentage increased. No residual estradiol valerate or metabolite was found in the meat of treated trout more than control trout.