The effects of oral administration of 3 mg of 17 alpha-methyltestosterone per kg of administered diet were studied in 35-day-old rainbow trout. The protein, fat, ash and moisture of meat, carcass efficiency, fillet efficiency and the hepatosomatic and gonadosomatic indexes were measured as well as the amount of residual methyl testosterone that might have remained in the meat. Compared to controls, there were no changes in crude protein and ash of the fish meat and in the fillet efficiency, carcass efficiency and gonodosomatic index of the trout. Crude fat of the fish meat and the hepatosomatic index decreased while the moisture increased. No residual 17 alpha-methyltestosterone was found in the meat of treated trout.