||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 parts per thousand) of the Caspian Sea. Fish weight at release was 2-3g (2.55 +/- 0.41g; 8.8 +/- 0.58cm 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 96hr 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 -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 96hr. Immunolocalization of NKCC indicated co-localization with NKA in the chloride cells in the gill epithelium. A partial sequence of the NKA -subunit (632bp) is described. Its expression levels were up-regulated at 12 and 48hr following salinity transfer (p<.05). However, NKA activity sharply increased in CSW specimens by almost 2.8-fold (p<.05) between 48 and 96hr 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.