Mechanisms of salinity acclimation in the euryhaline freshwater living teleost, Etroplus maculatus (BLEEKER), were studied. Brain water content decreased steadily with increasing salinity of the medium. Liver and muscle water content increased initially in 30% sea-water but in higher salinities decreased markedly, approximating control levels. Osmotic pressure of tissue fluid increased by 33% from 0.45% NaC1 equivalent in freshwater to 0.60% NaCl equivalent in t00~o sea-water. K and Mg decreased in dilute saline media but in t00% sea-water increased markedly approximating control levels; this is especially so in regard to Mg. Na decreased steadily and Ca increased with increasing salinity. C1 increased in 60~o sea-water but decreased in 100~o sea-water; however, its concentration in 100% sea-water exceeded that in freshwater. Free amino acids increased by 11% in 30% sea-water but decreased in 60~o and $00~o sea-water, such that their concentration was lower by 28O/o in 60% sea-water and by 34~o in ~00~o sea-water relative to the control level. Ascorbic acid decreased by 37~o in 30~o sea-water but in 60% and t00% sea-water increased steadily approximating the control level. The signffieance of these changes in the physiology of salinity acclimation is discussed and a correlation suggested between acclimation to salinity and to high temperature.