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The study tests the physiological responses of Persian sturgeon, Acipenser persicus, during the abrupt release of juveniles from freshwater ( FW) into brackish waters ( BW = 11‰) of the Caspian Sea. Fish weight at release was 2-3 g (2.55 ± 0.41 g; 8.8 ± 0.58 cm TL). Totals of 160 individuals were randomly distributed into four fiber-glass aerated tanks (volume 60-L). Two tanks served as controls ( FW groups), and two as exposure tanks for BW (Caspian Sea water = CSW). Fish were sampled at 0, 3, 6, 12, 24, 48 and 96 hr after abrupt transfer to CSW. Plasma osmolality, immunolocalization of Na+, K+ - ATPase ( NKA) and Na+/K+/2Cl- ( NKCC) Co-transporter, NKA activity and the NKA α-subunit mRNA expression were analyzed. Blood osmolality of fish transferred from FW to CSW increased significantly within hours post-transfer ( p < .05) and remained at a high level for up to 96 hr. Immunolocalization of NKCC indicated co-localization with NKA in the chloride cells in the gill epithelium. A partial sequence of the NKA α-subunit (632 bp) is described. Its expression levels were up-regulated at 12 and 48 hr following salinity transfer ( p < .05). However, NKA activity sharply increased in CSW specimens by almost 2.8-fold ( p < .05) between 48 and 96 hr after transfer. Gill NKCC co-transporter abundance increased, coinciding with increased gill NKA activity. The increased activity of NKCC during salt excretion in CSW may lead to an influx of Na+ into the chloride cells. Consequently, NKA activity increases to maintain intracellular Na+ homeostasis. [ABSTRACT FROM AUTHOR]