The levels of cortisol in blood serum and the Na+/K+ activated ATP-ase activity in the homogenates of the gills and the kidneys were measured in different species of acipenserids in freshwater (control) and after different periods of acclimation to brackish water of 12.5-12.7‰ salinity. Immature sturgeons (2+ years old) were used for the study: the sterlet Acipenser ruthenus L.- a freshwater species from the middle of the Volga River; the Siberian sturgeon from the Lena River A. baerii Brandt – a freshwater species that has short migrations to brackish water (5-10‰) of the estuary for feeding; the Russian sturgeon A. gueldenstaedtii Brandt and the starred sturgeon A. stellatus Pallas –both diadromous species of the Volga-Caspian basin that have regular migrations to brackish water (up to 14‰). The starred sturgeon was additionally held in 18‰ after 12.7‰. The investigated species from freshwater have similar levels of serum cortisol, but different dynamics of this hormone during exposure in brackish water. After seven days of the experiment, the level of serum cortisol increased (p<0.01) 4.1 times in the sterlet and 3 times in the Siberian sturgeon. In the Russian sturgeon after 7 days of experiment, cortisol level was similar to levels of control fish, although after 3 and 12 hours of acclimation (in the transition period to hypo-osmotic regulation) the hormone levels decreased (p<0.05). In the starred sturgeon, cortisol levels remained without changes during all periods of its life in brackish water. There was the increase of gill Na+/K+-activated ATP-ase activity only in the starred sturgeon exposed in water of 12.7‰ and 18‰ salinity. During acclimation to brackish water of 12.5-12.7‰ salinity, the enzyme activity did not change in the Siberian and the Russian sturgeons and decreased in the starlet. Levels of kidney Na+/K+-activated ATP-ase activity at this period decreased in studied species excluding the Siberian sturgeon that kept the enzyme activity without the changes. The conclusion was done that the changes in the serum cortisol levels and Na+/K+ ATP-ase activities in gills and kidneys after fish transfer to brackish water explain, to a great extent, the osmotic and ionic regulation capacities in different acipenserid species.