The present study was designed to investigate the effects of hydroxyl radicals (*OH), generated via the Fe2+-mediated Fenton reaction, on isolated rat aortic rings with and without endothelium. In the absence of any vasoactive agent, generation of *OH alone elicited an endothelium-independent contraction in rat aortic rings in a concentration-dependent manner. Hydroxyl radical-induced contractions of denuded rat aortic rings appeared, however, to be slightly stronger than those on intact rat aortic rings. The contractile responses to *OH were neither reversible nor reproducible in the same ring; even small concentrations of *OH radicals resulted in tachyphylaxis. Removal of extracellular calcium ions (Ca2+) or buffering intracellular Ca2+ with 10 microM acetyl methyl ester of bis(o-aminophenoxy) ethane-N,N,N',N',-tetraacetic acid (BAPTA-AM) significantly attenuated the contractile actions of *OH radicals. The presence of 1 microM staurosporine, 1 microM bisindolylmaleimide I, 1 microM Gö6976 [inhibitor of protein kinase C (PKC)], 2 microM PD-980592 (inhibitor of ERK), 10 microM genistein, and 1 microM wortmannin significantly inhibited the contractions induced by *OH. Proadifen (10 microM), on the other hand, significantly potentiated the hydroxyl radical-induced contractions. Exposure of primary cultured aortic smooth muscle cells to *OH produced significant, rapid rises of intracellular free Ca2+ ([Ca2+]i). Several, specific antagonists of possible endogenously formed vasoconstrictors did not inhibit or attenuate either hydroxyl radical-induced contractions or the elevation of [Ca2+]i. Our new results suggest that hydroxyl radical-triggered contractions on rat aortic rings are Ca2+-dependent. Several intracellular signal transduction systems seem to play some role in hydroxyl radical-induced vasoconstriction of rat aortic rings.