Pretreatment of bovine aortic endothelial cells with pertussis toxin inhibited partially the accumulation of inositol phosphates in response to ATP, whereas cholera toxin had no effect. Both pertussis and cholera toxins enhanced the stimulatory effect of ATP on prostacyclin release from the same cells. This action of cholera toxin was mimicked neither by an increase of cyclic AMP nor by the dissociated subunits of the toxin. Cholera and pertussis toxins, as well as aluminum fluoride, also potentiated the release of prostacyclin induced by ionophore A23187. These results suggest that a pertussis toxin-sensitive GTP-binding protein is involved in the coupling between P2-purinergic receptors and phospholipase C. In addition, another GTP-binding protein would play a crucial role at a further step in the control of PGI2 biosynthesis.