Ketanserin is a new strong antiserotoninergic drug that, unlike the previous ones, is selective for 5-hydroxytryptamine receptors. This drug has been employed successfully in the treatment of arterial hypertension and of some peripheral vascular diseases. The authors are carrying out a trial on medium term treatment with ketanserin (K) or propranolol (P) in comparison with placebo, to evaluate their effects on blood pressure, haemocoagulative parameters and peripheral circulation. The trial is a double-blind cross-over random trial on subjects with mild or moderate hypertension. Until now 13 patients have ended the study; six of them are suffering from arteriosclerosis obliterans of the lower limbs at 1st or 2nd stage according to Fontaine. Both propranolol and ketanserin significantly reduced the blood pressure, although the decrease in systolic blood pressure was more evident after propranolol. Heart rate diminished significantly only after propranolol administration. The noninvasive, intermittent (every 30 min) monitoring of blood pressure showed a significant 24-hour reduction of blood pressure after administration of propranolol or ketanserin without significant changes of circadian behaviour of the blood pressure. After administration of ketanserin a slight improvement in peripheral circulation was demonstrated, evaluated by using strain-gauge plethysmography. As regards the results obtained for platelet function and other haemocoagulative parameters examined, adenosine diphosphate-induced platelet aggregation, adenosine diphosphate slope, collagen lag period, antithrombin III biological activity, and serum fibrinogen did not show noticeable modifications after treatment, while beta-thromboglobulin levels decreased slightly after ketanserin administration.