The salinity tolerance and hydromineral regulation capabilities of juvenile great sturgeon, Huso huso, of different age and size groups (1: 35 days, BW: 0.60±0.02g &TL; 4.80±0.13clp; II: 35 days, BW: 2.23±0.20 g & TL: 7.19±0.17cm; III: 35 days, BW: 9.91±0.23g& TL: 11.86±0.24cm; IV: 50 days, BW: 3.97±0.25 g & TL: 9.34±0.18cm; V:50 days, BW:22.27±1.9g & TL: 17.25±0.35cm; VI: 65 days, BW: 6.3±0.67g& TL: 11.37cm ; n=90 for each group) in freshwater (FW:0.5% 0), estuary water (EW:9.5 %0)'and the Caspian Sea water (CsW:12.5% 0) were investigated. The fishes are directly transferred from FW to EW and CsW. The possible repercussions of osmoregulatory processes on some indicators of classical were examined at the end of 168 hours fish acclimation. Mortality was observed (higher than 50% after 72 hours) only in 1stgroup in EW and CsW. Some haematological parameters, namely haematocrit (Hct), red and white blood cells count (RBC, WBC), mean cell volume (MCV), hemoglobin concentration (Hb), mean cell hemoglobin concentration (MCHC), the amount of haemoglobin per erythrocyte (MCH), levels of cortisol, osmolarity and ion concentration (Na+, K+, .16 S.M. Farabi et ai., Ca+2 and Mg+2) in the plasma were determined. The functional levels of the mechanism of osmotic and ionic homeostasis were similar in different groups (II to VI) but differed in experimental media (P<0.05). Significant differences were observed between the levels of serum Na+ concentration in different groups in EW and CsW media (p<0.05). Serum Na+ and Ca+2 concentrations were higher than those of FW and EW media, but lower than in CsW media. The Hct, MCV and MCH decreased with increase in fish age, and decreased from FW media to CsW media with increasing salinity; RBC, WBC and MCHC did not change. The trend of increasing levels of cortisol were observed at higher salinity in each group (P<0.05). However, Hct, MCV, MCHC, cortisol, osmolarity and ion concentration values did not return to initial values (P<0.05), showing that osmoregulatory processes caused major physiological changes in this species.