The present experiments were designed to investigate the effects of omeprazole, a H+-K+ ATPase inhibitor, on corporal smooth muscle tone in vitro. All spontaneous contractile activity in the corpus cavernosum was blocked following omeprazole (0.1 mM-1 mM) administration. However atropine (1 mu M), N-w-nitro L-arginine methyl ester (L-NAME, 30 mu M) or indomethacin (10 mu M) did not affect the spontaneous contraction. Omeprazole (10 mu M-1 mM) concentration-dependently induced relaxation in corporal smooth muscle precontracted with 10 mu M phenylephrine or 80 mM KCI. Pretreatment of corporal tissue with L-NAME (30 mu M) indomethacin (10 mu M), ammonium chloride (7.5 mM), sodium acetate (7.5 mM), tetraethyl ammonium chloride (0.5 mM) or glibenclamide (1 mu M) had no effect on the omeprazole induced relaxant responses. Nimodipine, an L-type Ca++ channel blocker relaxed corporal strips precontracted with 80 mM KCI. Collectively, these results indicate that the inhibition of spontaneous Contraction and the relaxation of precontracted corporal smooth muscle by omeprazole is probably mediated by the blockade of calcium channels. Further work is needed to determine the cellular mechanism(s) of action by which omeprazole acts on corpus cavernosum smooth muscle.