K+ channels regulate tone in both the systemic and pulmonary circulations. K+ channel inhibition leads to membrane depolarization, Ca2+ influx and vasoconstriction; K+ channel activation leads to hyperpolarization and vasodilatation. The sulfhydryl oxidant diamide opens K+ channels in pulmonary smooth muscle and acts as a potent vasodilator in perfused lungs. We examined the hypothesis that antioxidants cause constriction and oxidants cause relaxation through their effects on K+ channels in vascular smooth muscle. The oxidant diamide (380 microM and 3.8 mM) inhibited the reduction of cytochrome C by ferrous sulphate in vitro whilst the antioxidants co‐enzyme Q10 (770 microM) and duroquinone (700 microM) increased the rate of reduction. Both antioxidants caused dose‐dependent constriction of endothelium‐intact and ‐denuded rat pulmonary artery and aortic rings. This constriction could be reversed by 1 microM diamide. Co‐enzyme Q10 and duroquinone (both at 100 microM) partially inhibited (approximately 30%) whole‐cell K+ channel currents and depolarized membranes of isolated pulmonary artery smooth muscle cell recorded using the amphotericin‐perforated‐patch‐clamp technique. Diamide (100 microM) increased whole‐cell K+ channel currents and hyperpolarized the membrane. The data suggest that oxidants and antioxidants may modulate vascular tone via an effect on K+ channels.