A total of 1,500 larvae of Nile tilapia was distributed in 15 20-L aquaria (100 larvae in each one) to compare two methods of masculinization: via oral application, using a diet with hormone (60 mg 17α-methyltestosterone.kg-1); and through immersion bath (6 mg 17α-metyltestosterone.L-1), each one with five replicates. Larvae and juvenile were sampled on day 1 (beginning of the experiment) and on days 30 (end of hormone feeding period), 40, 45, 60 and 90. One sample with 0.5 g of fish was collected from each replication for analysis of body testosterone. Fish fed diet with hormone were given experimental ration for 30 days and commercial ration until the end of the experiment, and fish in immersion bath received commercial ration and they were submitted to immersion bath (6 mg 17 α-metyltestosterone.L-1) for 36 hours on days 6 and 10 after the beginning of the experiment. For fish given ration without hormone (control), values of body testosterone were almost totally steady over the experiment, moderately increasing from day 60. Concentrations of body testosterone in fish fed diet with hormone or immersion bath were the highest on day 30. For fish submitted to immersion bath, the values were reduced on day 40 and they increased again until 60 days of observation, while for those submitted to diet with hormone, concentrations of testosterone gradually increased until 60 days. The use of 17 α-methyltestosterone through oral administration or immersion bath of larvae promotes sexual maturation of fish from day 45, especially on those fed diet with hormone. Concentrations of hormone in the carcass are lower than the recommended by Codex Alimentarius from Brazil as safe for human consumption.
