N-formylmethionyl-leucyl-phenylalanine (FMLP), a chemotactic tripeptide, is known to cause intracellular alkalinization. Moreover, there is a specific receptor for FMLP in vascular endothelial cells but not in vascular smooth muscle cells. Because we have already reported that intracellular alkalinization inhibits acetylcholine (ACh)-induced relaxation, we examined whether FMLP alters the vasodilation of endothelial cells through intracellular alkalinization. FMLP reduced ACh-induced relaxation in aortic rings from Sprague-Dawley rats but did not affect nitroglycerin-induced relaxation. N-t-butoxycarbonyl-phenylalanyl -D-leucyl-phenylalanyl-D-leucyl-phenylalanine, a specific formyl receptor antagonist, reversed the impairment of ACh-induced relaxation, as did the protein kinase C inhibitors sphingosine and 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine (H-7). The sodium/proton antiport inhibitor amiloride and the proton ionophore nigericin normalized the attenuated ACh-induced relaxation. FMLP-induced impairment was normalized by the phospholipase A2 inhibitor quinacrine, the cyclooxygenase inhibitor indomethacin, and the antagonists of the prostaglandin H2 and/or thromboxane A2 receptor, ONO-3708 and S-1452, respectively. Superoxide dismutase inhibited the effect of FMLP. In conclusion, FMLP attenuated ACh-induced relaxation, possibly through intracellular alkalinization. Increased production of vasoconstrictor prostaglandin(s) and superoxide may contribute to the inhibitory effect of FMLP-induced alkalinization on ACh-induced relaxation.