Thrombin induced an increase in [Ca 2+]i in mouse mastocytoma P-815 cells. This increase was markedly reduced by prior exposure to pertussis toxin (PT) but not by removal of extracellular Ca 2+, suggesting that thrombin stimulates phospholipase C via a PT-sensitive GTP-binding protein. ATP also induced an increase in [Ca 2+]i. This increase was insensitive to PT but completely suppressed on removal of extracellular Ca 2+, suggesting that ATP stimulates Ca 2+ influx in a PT-insensitive manner. Iloprost, a stable prostacyclin analogue, increased the cellular cAMP level and dose-dependently inhibited the thrombin-induced increase in [Ca 2+]i, whereas the ATP-induced increase in [Ca 2+]i was markedly enhanced by iloprost. Cyclic AMP analogues, dibutyryl cAMP and 8-bromo cAMP, also inhibited the increase in [Ca 2+]i induced by thrombin and promoted that by ATP, indicating that the inhibitory and stimulatory effects of iloprost are mediated by cAMP. These results suggest that the prostacyclin receptor differentially regulates two distinct Ca 2+ mobilizing systems via cAMP in mastocytoma cells.