Clonidine hydrochloride (Catapres), a potent antihypertensive agent, has been in clinical use since 1974 in the United States. Clonidine, an alpha-adrenergic receptor agonist, stimulates central alpha receptors in the depressor site of the vasomotor center of the medulla oblongata and hypothalamus, which diminishes efferent sympathetic tone to the heart, kidneys, and peripheral vasculature with a concomitant increase in vagal activity. Hemodynamic and renal effects include reduction in supine and erect blood pressure, heart rate, total peripheral resistance, plasma renin activity, and urinary aldosterone and catecholamine excretion, with little effect on resting cardiac output, response to exercise, and preservation of renal function. Clonidine alone produces a significant reduction in mean arterial pressure in all degrees of hypertension during acute and chronic administration, with little or no tendency toward tolerance or postural hypotension. Its antihypertensive potency is enhanced with the concomitant use of a diuretic or vasodilator, and it may be used in place of a beta blocker with equal efficacy in the diuretic plus vasodilator combination. Serious adverse effects are uncommon, with more than 93% of patients tolerating the drug well. Sedation and dry mouth, the most common adverse effects, are usually related to dose and duration and are minimized by gradually increasing the dose and by taking the major portion of the twice-daily schedule at bedtime. Clonidine may be safely given to patients with congestive heart failure, ischemic heart disease, obstructive lung disease, chronic renal insufficiency, and diabetes mellitus. Clonidine is one of the most versatile and effective agents presently available for the treatment of hypertension.