In this study, in order to determine the appropriate time and size to release Salmo trutta caspius, in Kelardasht center conditions, some biometric and physiological parameters were investigated. In this regard, sampling was carried out twice in each season from Parr (one-summer fingerlings) to smolt (two- summer fingerlings) stages in the course of 1 year. The trend of length, weight and condition factor, ions, Osmolality, thyroid hormones (T3, T4) and Cortisol levels of plasma in Freshwater fish and those transferred to Caspian sea Brackish water were measured. Based on the results, weight and length of fishes increased significantly from 8 g and 9.2 cm in autumn to 35.3 g and 14.9 cm in next summer, respectively (p<0.05). Ions and Osmolality levels did not show significant differences among different seasons, but after transfer to Caspian Sea water a significant increase in ions levels was observed. The lowest and highest levels of T3 observed in autumn and summer, respectively, and difference between these seasons were significant. The highest level of T4 was recorded in spring which was significantly different from that value of autumn. Plasma T4 levels showed significant differences between Freshwater and Brackish water in autumn. There were no significant differences among different seasons and Fresh and Brackish water, but the least and highest levels of Cortisol observed in spring and autumn, respectively. Based on these results, it seems that the fish with 20.9 g weight and 12.9 cm length in spring has the most appropriate physiological conditions for release.