The salinity tolerance, hematological and hydromineral regulation capabilities of juvenile Russian sturgeon (Acipenser gueldenstaedtii) were investigated in different ages and sizes in freshwater (FW: <0.5‰), estuary water (EW: 9.5‰) and Caspian Sea water (C_SW: 12.5‰). The fish were directly transferred from FW to EW and C_SW. The possible repercussions of ion-osmoregulatory processes on some classical indicators were examined at the end of 168 hours fish salinity tolerance. The mortality was not more than 35% in EW and C_SW in both groups. The survival percent and salinity tolerance were increased with increment of age and size of fish. The functional mechanism of osmotic and ionic homeostasis were similar in all groups (p>0.05) but differed in experimental media (p<0.05). Signifi cant differences were observed between the levels of plasma ion concentrations in different media (p<0.01). Plasma Na⁺, K⁺ and Ca⁺² concentrations were higher than those of FW media, but lower than in C_SW media (p<0.05). Plasma Mg⁺² concentrations were lower than those of FW and C_SW media, but near to EW media (p<0.05). The hematocrit mean cell volume decreased but mean cell hemoglobin concentration, osmolarity and levels of plasma cortisol were increased from FW media to C_SW media (p<0. 05). The hemoglobin concentration (Hb), the amount of hemoglobin per erythrocyte, red and white blood cells count did not change (p>0.05). However, the results showed that the above parameters in fish fingerlings did not return to initial values in the new environment and then physiological changes happened.