The 17 alpha-Methyltestosterone treats carried out in two periods. The Rainbow trout fingerlings (6.43 +/- 0.31 g.) were fed 2.5 and 5 mg/kg MT in first period, lasted 92 days. 2.5 mg/kg MT in second period lasted 96 days. Generally in the this study, 17 alpha-MT was treated 2.5 mgMT/kg-diet, throughout 92 days, and 188 days and 5 mgMT/kg-diet, throughout 92 days to rainbow trout. The group of 2.5 mgMT/kg-diet was separate two groups and then one of them was fed with 2.5 mgMT/kg-diet go on in the second period, lasted 96 days. Other group was fed with control food. Normal feed without MT was given to the control fishes. As a consequence of the analysis, it was found out that MT does not cause a change in the crude protein and crude ash (p>0.05), however it increased the values of the crude fat and decreased the value of moisture (p<0.05) of fish-meat. It not effected the fillet efficiency (p>0.05), however it effected percent of internal organ and visceral fat content and HSI (p<0.05). In the fish, treated 2.5 mg MT/kg-diet, throughout 188 days, visceral fat and percent of internal organ were decreased.
