Transdermal administration of antihypertensive agents such as clonidine is an attractive concept, for it allows therapeutically effective constant concentrations of drug to be maintained in the plasma with comparatively low total doses. Blood pressure can be controlled without the need for the larger drug doses associated with oral therapy, and adverse effects are thereby minimized. Clinical studies with transdermal clonidine applied once weekly have established its efficacy during prolonged treatment periods in hypertensive patients. Approximately 60% of patients with mild to moderate hypertension have their blood pressures controlled with this therapy, which works equally well in white and in black patients. Symptomatic side effects are fewer and milder than with corresponding oral therapy, but 10% to 15% of patients develop local skin reactions to the transdermal systems. These reactions may reflect simple irritation or may be allergic in nature; in either case, they tend to disappear rapidly if the treatment is withdrawn, and there is very little evidence for systemic consequences. Transdermal clonidine is effective in elderly patients and its action does not seem to be associated with deleterious effects on cerebral blood flow in these individuals. Other studies with this treatment have confirmed an absence of adverse effects on glucose metabolism in diabetic patients or on renal function in patients with kidney disease. Endocrinologic and metabolic effects with transdermal clonidine are similar to those with the oral form: norepinephrine, renin, and aldosterone all tend to be reduced; and there are no significant changes in the lipid profile, uric acid, or electrolytes.(ABSTRACT TRUNCATED AT 250 WORDS)