Amphiphilic peptides with positive charges such as substance P (SP) and mastoparan (MP) are known to induce exocytosis in rat peritoneal mast cells. To elucidate whether and how intracellular Ca 2+ signaling is involved in the peptide-induced exocytosis, here we investigated the relationships between an increase in intracellular free Ca 2+ concentration ([Ca 2+] i) and exocytosis caused by SP and MP. SP and MP induced exocytosis coinciding with an initial rapid and transient [Ca 2+] i increase, but not with a sustained increase. These stimulations were abolished by pertussis toxin, indicating the involvement of the G i-family of G proteins in the peptide signaling. Moreover, the [Ca 2+] i increase was shown to accelerate and potentiate exocytosis, suggesting that the transient increase in [Ca 2+] i positively modified exocytotic secretion. However, it was indicated that the signal of [Ca 2+] i increase was not sufficient for the peptide-induced exocytosis, suggesting the participation of alternative mechanisms other than Ca 2+ signaling in the pathway.