The effect of different growth conditions on the activity of the Na(+)/H(+) antiporter in Dunaliella salina has been investigated. Adaptation of D. salina cells to ammonia at alkaline pH or to high NaCl concentrations is associated with a pronounced increase in the plasma membrane Na(+)/H(+) exchange activity. The enhanced activity is manifested both in vivo, by stimulation of Na(+) influx into intact cells in response to internal acidification, and in vitro, by a larger (22)Na accumulation in plasma membrane vesicles in response to an induced pH gradient. Kinetic analysis shows that the stimulation does not result from a change of the K(m) for Na(+) but from an increase in the V(max). In contrast, adaptation of cells to a high LiCl concentration (0.8 m) depresses the activity of the Na(+)/H(+) antiporter. Adaptation to ammonia is also associated with a large increase of three polypeptide bands in purified plasma membrane preparations, indicating that they may compose the antiporter polypeptides. These results suggest that adaptation to ammonia or to high salinity induces overproduction of the plasma membrane Na(+)/H(+) antiporter in Dunaliella.