The changes in osmolarity and ion concentrations of blood serum, urine and intestinal liquid in different acipenserid species were studied during their acclimation to sea water, identifying the osmotic and ionic regulation involved in these species. The freshwater species (Acipenser ruthenus, A. baerii from Baikal Lake) can adapt to brackish water of 12.5‰ salinity as a conformer. The freshwater species Pseudoscaphyrhinchus kaufmanni is also an osmo-conformer, but tolerates brackish water up to 10‰ salinity only. Freshwater species having short migrations to the estuary for feeding (A. baerii from Lena River) have limited capacity in osmotic and ionic regulation. The diadromous brackishwater species (A. gueldenstaedtii, A. stellatus, Huso huso) held in brackish water of 12.5-12.7‰ salinity and diadromous seawater species (A. brevirostrum and A. oxyrhynchus) held in sea water of up to 30.5‰ are capable to support blood serum osmolarity at relative constant levels during the transition phase from hyper- to hypo-osmotic regulation after the transition from fresh water. However functional level of the mechanism of osmotic and ionic homeostasis is not equal in different species. The higher the medium salinity (where species live in the nature) the higher the functional level of these mechanisms. This level rises in a number of acipenserids which rank as follow “Pseudoscaphyrhinchus kaufmanni → A. ruthenus and A. baerii from Baikal Lake → A. baerii from Lena River→ A. gueldenstaedtii→ Huso huso→ A. stellatus→ A. brevirostrum→ A. oxyrhynchus”. The development of osmotic and ionic homeostasis is connected with the evolution of kidney function (the increase of sodium re-absorption and magnesium secretory activities), with the strengthening sodium absorption function of the intestines, with the strengthening of ionic secretory function of the gills and the changes regulatory function of endocrine system.