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This study was designed to better characterise in humans the mechanism of action of rilmenidine, a new antihypertensive oxazoline derivative. The functional relationship between hypotensive response, adrenergic nerve activity and platelet α2-adrenoceptor number and function was evaluated in hypertensive subjects, both before and after prolonged treatment with this drug. The effects of rilmenidine were compared with those induced by a diuretic drug (hydrochlorothiazide). After 1 month of placebo, 24 patients with essential hypertension were randomly allocated to either rilmenidine 1mg once daily or hydrochlorothiazide 25mg once daily. At the end of the first month of treatment, the 2 drugs were combined in the patients whose diastolic blood pressure was higher than 90mm Hg. On days 0, 30 and 60, the number and affinity of blood platelet α2-adrenoceptors were measured by 3H-yohimbine labelling, and plasma catecholamines were determined. On days 0 and 30, platelet aggregation was also assessed.The results show that the administration of rilmenidine exerts significant and lasting antihypertensive activity. This effect, however, is not associated with a modification in platelet α2-adrenoceptor density and function, nor does it seem to affect catecholamine plasma levels, thus suggesting a negligible role for α2-adrenoceptor stimulation in the antihypertensive action of rilmenidine.