Fluorescent probes were used to visualize the morphology of membranes and of F-actin in rat peritoneal mast cells, exposed to hyperosmotic medium and consequently reversed to isotonicity. Hypertonicity induced cell shrinkage followed by a regulatory volume increase, and cell alkalinization that was sensitive to amiloride, an inhibitor of the Na+/H+ exchanger (NHE), but not to Latrunculin B, an inhibitor of actin polymerization. Using Bodipy-Sphingomyelin, we have observed formation of vacuole-like dilations (VLDs), primarily at or close to the adhesion plane, following the reversal from hyper- to isotonic medium. VLD formation was not inhibited by Latrunculin B or by amiloride. Phalloidin staining has shown that actin filaments do not surround the vacuoles and latrunculin-induced depolymerization of actin has actually promoted vacuole formation, even in isotonic conditions. The results support the idea that a decrease in membrane tension promotes the internalization of the plasma membrane.