In this study, efficient hormone protocol for establishment of masculinized female brood stock, which was used in monosex female rainbow trout (Oncorhynchus mykiss) production, was investigated. Starting from first feeding, larva that was going to be used as brooders fed formula diets containing 1, 2 and 3 mg 17 alpha-methyltestosterone (MT) for 600 degree-days. Otherwise, sac fry at various ages were immersed once and 2 to 6 times into 0.5 mg l(-1) MT baths for 2 h with 48 h intervals. While oral MT treatments were effective to produce all or almost all male populations at all 3 dosages, none of the bath treatments achieved a similar success and most yielded only a few intersex individuals. Control of oral administration fish at 16 months of age showed that fish masculinized by giving the highest MT dosage contained lower ratio of functional males with complete sperm ducts than fish masculinized by giving the lowest MT dosage. Additionally, high MT dosage was determined to have adverse effect on gonad size. In conclusion, the results of the study showed that oral MT administrations produce more successful and consistent results for the production of nnasculinized female brooders. They demonstrated that high MT dosages can adversely affect both sperm duct development and gonad size in sex reversed fish. Furthermore, it was discussed that the actual dosages of MT that larva were exposed could vary at different water temperatures depending on variable metabolic rates.