Previously, we suggested that myosin light chain kinase (MLCK) is involved in shrinkage-induced activation of the Na+/H+ exchanger in rat astrocytes. Here we have studied the effects of hyperosmotic exposure in C6 glioma cells, a common model for astrocytes. Shrinkage-induced activation of the Na+/H+ exchanger in C6 cells is directly proportional to the degree of shrinkage, results in an alkaline shift in the pK' of the exchanger, is dependent on ATP, and is inhibited by ML-7 (an MLCK inhibitor) and by various calmodulin inhibitors. Cell shrinkage also results in increased phosphorylation of myosin light chain (MLC). Interestingly, shrinkage-induced activation of the exchanger does not occur in subconfluent C6 cells. However, phosphorylation of MLC still occurs in subconfluent cultures of C6 cells on shrinkage, suggesting that the lack of activation in these cells occurs at a point between MLC phosphorylation and Na+/H+ exchange activation. The lack of activation of Na+/H+ exchange in subconfluent C6 cells can be utilized to further elucidate the shrinkage-induced activation pathway.