The electrophysiological and mechanical properties of the vasodilator minoxidil sulfate (MNXS) were examined in isolated smooth muscle cells and strips from rabbit portal vein. At micromolar concentrations, MNXS inhibited norepinephrine (0.1–1.0 μM) -induced contractionsin isolated muscle strips. In isolated cells, norepinephrine caused a dose-dependentdepolarization of the resting membrane potential, which was significantly attenuated by MNXS(5 μM); MNXS alone caused a hyperpolarization of the membrane potential. This hyperpolarizationwas insensitive to Na+-K+pump blockade by ouabain, but was inhibited by the K+channel antagonist, tetraethylammonium (20 mM). In voltage-clamp experiments, a resting(background) conductance associated with the resting membrane potential was identified. Thisconductance, which previously has been shown to be reduced by Ba2+as well as tetraethylammonium, was increased by MNXS (2 μM). In additional experiments, whole-cell L-type Ca2+currents were inhibited by micromolar concentrations of MNXS. These experiments show thatconcentrations of MNXS that inhibit norepinephrine-induced contractions promote K+conductanceand inhibit Ca2+entry through voltage-dependent Ca2+channels in vascular smoothmuscle cells. These electrophysiological effects of MNXS may be responsible for the vasorelaxanteffects of the drug observed in vitro and in vivo.