We investigated the effect of omeprazole (1 x 10(-5) -3 x 10(-4) M), an inhibitor of H+,K+-ATPase, on rat aortic rings pre-contracted with phenylephrine (10(-6) M). Omeprazole relaxed the tissue in a concentration-dependent manner. Either removal of the endothelium or incubation with nitric oxide (NO) synthase inhibitor N-G-nitro-L-arginine methyl ester (L-NAME, 3 x 10(-5) M) significantly attenuated the relaxations. Pre-treatment with L-arginine (10(-3) M), but not with D-arginine, reversed the inhibitory action Of L-NAME. Indomethacin (10(-6) M) and tetraethylammonium (TEA, 10(-2) M) did not affect the relaxant responses to omeprazole indicating the lack of involvement of cyclooxygenase products and K+ channels, respectively. These results suggest a role of NO in the mechanism of action of omeprazole.