The green sturgeon is a long-lived, highly migratory species with populations that are currently listed as threatened. Their anadromous life history requires that they make osmo- and ionoregulatory adjustments in order to maintain a consistent internal milieu as they move between fresh-, brackish-, and seawater. We acclimated juvenile green sturgeon (121 +/- 10.0 g) to 0 (freshwater; FW), 15 (estuarine; EST), and 24 g/l (SF Bay water; BAY) at 18 degrees C for 2 weeks and measured the physiological and biochemical responses with respect to osmo- and ionoregulatory mechanisms. Plasma osmolality in EST- and BAY-acclimated sturgeon was elevated relative to FW-acclimated sturgeon (P < 0.01), but there was no difference in muscle water content or abundance of stress proteins. Branchial Na(+), K(+)-ATPase (NKA) activity was also unchanged, but abundance within mitochondrion-rich cells (MRC) was greater in BAY-acclimated sturgeon (P < 0.01). FW-acclimated sturgeon had the greatest NKA abundance when assessed at the level of the entire tissue (P < 0.01), but there were no differences in v-type H(+)ATPase (VHA) activity or abundance between salinities. The Na(+), K(+), 2Cl(-) co-transporter (NKCC) was present in FW-acclimated sturgeon gills, but the overall abundance was lower relative to sturgeon in EST or BAY water (P < 0.01) where this enzyme is crucial to hypoosmoregulation. Branchial caspase 3/7 activity was significantly affected by acclimation salinity (P < 0.05) where the overall trend was for activity to increase with salinity as has been commonly observed in teleosts. Sturgeon of this age/size class were able to survive and acclimate following a salinity transfer with minimal signs of osmotic stress. The presence of the NKCC in FW-acclimated sturgeon may indicate the development of SW-readiness at this age/size.