Tolmesoxide is a member of a series of aryl sulphoxides, which were synthesized as potential anti-hypertensive agents. Studies in rats indicate that tolmesoxide lowers blood pressure by a direct relaxant effect on vascular smooth muscle. Tolmesoxide is approximately equipotent in dilating arterioles and noradrenaline constricted veins. Thus, tolmesoxide should be classified as a non-selective vasodilator. Tolmesoxide, in common with other vasodilators, has a short half-life, probably due to rapid metabolism as indicated by the early appearance of the major metabolite in the plasma. However, the relatively long duration of action (up to 12 h) contrasts with the short half-life and suggests that tolmesoxide has a specific mechanism of action of relatively long duration in the vascular walls in a manner suggested for hydralazine and minoxidil. The bioavailability of oral tolmesoxide from capsules averaged 84.5% and was independent of dose. Described side effects are: facial flushing, severe nausea, headaches and palpitations.