Mastoparan, a peptide toxin from wasp venom, stimulated the accumulation of inositol phosphates in rat parotid acinar cells. Addition of this peptide to fura-2-loaded cells resulted in a rapid increase in the fura-2 fluorescence ratio (340 nm/380 nm), suggesting that mastoparan stimulates an increase in cytosolic Ca2+ concentration. However, this change in the ratio appears to be due, in part, to fura-2 leakage from the cells, because addition of Mn2+, which quenches extracellular fura-2 fluorescence, reduced the increased fluorescence ratio. In addition to the fura-2 leakage, mastoparan caused considerable leakage of lactate dehydrogenase, a cytosolic marker enzyme. Furthermore, mastoparan decreased the number of trypan blue-excluding cells, indicating a decrease in cell viability. These results suggest that mastoparan enhances the membrane permeability by a mechanism independent of the activation of G-proteins.