Mastoparan, a tetradecapeptide toxin from wasp venom stimulates secretion in mast cells and enhances GTPase activity of several purified guanine nucleotide regulatory proteins (G proteins). This suggests that this toxin may effect cellular functions through activation of G proteins. In this report, we probed the effects of mastoparan on cytosolic calcium concentration ([Ca2+]i) and inositol trisphosphate (IP3) formation in human polymorphonuclear leukocytes (PMN). At noncytotoxic concentrations up to 35 microM, mastoparan induced a dose-dependent elevation in [Ca2+]i in PMN, as determined by the fluoroprobe Fura 2. The increase in [Ca2+]i was attained through two discrete processes involving an initial rapid and transient calcium rise followed by a slower sustained increase. The initial but not the second [Ca2+]i increase was absent in PMN pretreated with pertussis toxin. The second but not the first [Ca2+]i rise required external calcium. The kinetics of [Ca2+]i changes and dependency on extracellular calcium induced by mastoparan correlated with the production of IP3. Pertussis toxin inhibited only the initial phase of IP3 production. The ability of pertussis toxin to ADP-ribosylate Gi-like proteins in PMN membrane was potentiated in the presence of mastoparan. Thus, mastoparan activates phospholipase C in PMN through two independent mechanisms. The first pathway is similar to that induced by chemoattractant receptors in that the rapid and transient activation of phospholipase C is dependent on a pertussis toxin-sensitive Gi protein. The second pathway is delayed, sustained, insensitive to pertussis toxin, and requires extracellular calcium.