Chalcalburnus tarichi is an anadromous cyprinid fish that has adapted to extreme conditions (salinity 22 aEuro degrees, pH 9.8 and alkalinity 153 mEq x l(-1)) in Lake Van in eastern Turkey. Changes in immunoreactivity of Na+/K+-ATPase in gill tissue and osmolarity and ion levels in plasma were investigated in C. tarichi during reproductive migration. Physicochemical characteristics and ion levels in Lake Van were high compared freshwater. Plasma osmolality and plasma ion concentrations ([Na+], [K+] and [Cl-]) increased after transfer from freshwater to Lake Van. The mitochondria-rich (MR) cells of the gill were stained in both filament and lamellar epithelia of C. tarichi by immunocytochemistry with a specific antiserum for Na+/K+-ATPase in river fish samples. Density and area of MR cells were decreased in lake-adapted fishes. These results indicated that freshwater acclimation capacity is correlated with the size and distribution of MR cells in C. tarichi, in contrast to many teleost fishes.