The addition of porcine pancreatic group I phospholipase A2 (PLA2-I) produced a transient contraction followed by a relaxation in helical strips of porcine cerebral arteries. Its ED50 value (2.3 nM) was almost identical to the Kd value (3.9 nM) calculated from the specific binding of 125I-labeled porcine PLA2-I in cultured porcine cerebral arterial smooth muscle cells. Type-specific action of PLA2s and homologous desensitization strongly implicated the involvement of PLA2-I-specific sites in the response. The transient contraction was abolished by treatment with indomethacin as well as by the removal of endothelium, indicating the dependence of vasoconstrictor prostaglandins synthesized by PLA2-I in endothelium. The PLA2-I-induced relaxation response was also observed in bovine and cat cerebral arteries, thus providing a new aspect of PLA2-I as a vasoactive substance.