_^ifferent doses of 17 a-methyltestosterone hormone (MT) used as I) growth promoter was administrated to Nile tilapia; Oreochromis niloticus in fishmeal based pelleted diet for 90 days. The obtained results showed that low doses of 0.5, L0 and 2.5 mg MT/kg feed were not effective meanwhile the dose of 5.0 mg/kg was the optimum effective one in promoting significant final weight, weight gain and SGR of Nile tilapia. HSI was insignificantly changed at low MT doses, and slightly increased at high MT doses. Male and female GSI was significantly decreased at high MT doses (10, 20 and 40 mg/kg), while insignificant changes were observed at low doses. Feed intake and FCR were slightly changed at different doses of MT. The higher PER was obtained with moderate MT doses and the least ones were obtained with 20, 40 mg/kg or control. The changes in erythrocyte counts, haemoglobin content, haematecrit value and plasma glucose showed insignificant differences at all treatments. A significant reduction of plasma total protein was observed in fish fed 40 mg MT/kg, whereas it was insignificantly changed with other treatments. in contrast, the highest level in plasma total lipids was obtained at 40 mg MT/kg. In fish plasma, the activity of ALT was the highest with control fish and that fed low doses of 0.5-2.5 mg MT/kg, while the least one was obtained with 40 mg/kg. In contrast, AST activity was significantly increased with high MT doses of 20 and 40 mg/kg, while there were no significant changes among other treatments. The higher activity of alkaline phosphatase (ALP) was obtained at low MT doses (0 to 2.5 mg/kg), after which ALP activities decreased to reach the lowest one at 40 mg/kg. Hepatic ALT and AST activities were increased with increasing MT doses where the maximum ALT and AST activities were obtained at 40 mg/kg, while the minimum ones were obtained at MT doses of 0 to 5 mg/kg. In fish muscle, the activity of ALT and AST showed significant reduction with increasing MT doses where the minimum one was obtained with 40 mg/kg. On the other hand, AST activity was insignificantly changed at all treatments.