The activity of the Na+/H+ exchange system of rat thymic lymphocytes was determined by means of intracellular (pHi) and extracellular pH (pH0) measurements. In isotonic media, the antiport is virtually quiescent at physiological pHi (7.0-7.1), but is greatly activated by cytoplasmic acidification. At normal pHi, the antiport can also be activated by osmotic shrinking. Osmotic activation occurs after a delay of 20-30 s and is reversed several minutes after iso-osmolarity is restored. The mechanism of activation was analyzed by comparing the kinetic parameters of transport in resting (isotonic) and hyperosmotically stressed cells. The affinities of the external substrate site for Na+ and H+ are not altered in shrunken cells. In contrast, the Hi+ sensitivity of the antiport (which is largely dictated by an allosteric modifier site) was increased, which accounted for the activation. The concentration of free cytoplasmic Ca2+ [( Ca2+]i) increased after osmotic shrinking. This increase was dependent on the presence of extracellular Ca2+ and Na+ and was blocked by inhibitors of Na+/H+ exchange, which suggests that it is a consequence, rather than the cause, of the activation of the antiport. It is concluded that the shift in the pHi dependence of the modifier site of the Na+/H+ antiport is the primary event underlying the regulatory volume increase that follows osmotic shrinkage.