This study was carried out to determine the appropriate size of Caspian trout (Salmo trutta caspius Kessler, 1877) juveniles for releasing to South Caspian Sea or possibility of cage culture in Caspian Sea water. 1611 specimens were exposed in 4 weight groups of 5, 10, 15 and 20 g, in 3 salinity trials: Caspian Water (11- 11.5), inshore water (7) and fresh water (control). Each trial was done in 3 replicates. The blood samples and tissue fixations carried out from juveniles of control group (in fresh water) and 3, 6, 12, 24, 72, 168, 240 hours after exposure of fish in different treatments. Plasma osmolal ity, Na^+ and Cl^- concentrations, were measured by osmometer, flame photometer, RA1000 respectively. Plasma cortisol level was determined by using RIA (radio immunochemical assay). Na^+, K^+-ATPase activity in homogenates of gills was estimated by phosphate released from ATP. Histological indicators including chloride cell diameter and nephron morphometric parameters were assessed using classic preparation and optic microscope with digital camera. Results of osmolality and ions measuring concurrently show that all weight groups can live in salinity of 7 and they maintain the osmolality and ion concentrations. In the Caspian water, weight groups excluding 5 g juveniles show same result. Mean plasma osmolality of 20, 15, 10 and 5 gr juveniles in control group (time of 0) were calculated 331.3±8.7, 307.7±6, 334.7±14.6 and 301±8.7 mosml/l. This parameter in the above weight groups after 240 hours exposure in the Caspian Sea water were measured 329±0.53, 321±9, 325.3±6.7 and 346.5±13.6 respectively. The observation of kidney glomeruli in histological sections shows that the diameter of glomeruli in 5, 10, 15 g weight groups in 7 and all groups in Caspian water decreased after 72 h adaptation period (p0.05) for 5g juveniles, whereas within weight groups of 10, 15 and 20 g in Caspian Sea water and groups of 15, 20 g in water of 7 salinity, the increase (p0.05), although in other groups, a significant increase of this parameter was detected during experiments. Na^+ ,K^+ -ATPase activity in juveniles of 5g weight group in 7 salinity and Caspian water was low (3.2 6.1 mol Pi /mg protein/ h). The enzyme activities in all weight groups were higher under the exposure in Caspian Sea water than that in water of 7 salinity. In group of 10 g juveniles at start time (control in freshwater) the activity of Na^+, K^+ -ATPase was significantly higher (p<0.05) than that in 20g group. It is may be related to some metabolic changes and transforming to parr-smolt
