The antihypertensive effect of clonidine hydrochloride delivered at a constant rate for seven days by transdermal disks was evaluated in seven patients with essential hypertension. Blood pressure values measured at the physician's office were not significantly decreased by one month of treatment with one (n = 2) or two (n = 5) once-weekly applied clonidine transdermal disks. In contrast, blood pressure values recorded during patients' customary daily activities by means of a portable blood pressure recorder were considerably reduced, from 159/97 +/- 2/2 to 136/76 +/- 7/5 mm Hg. Plasma drug concentration at the end of the fourth week averaged 1.22 +/- 0.24 ng/mL. Plasma renin, vasopressin, and epinephrine levels were not modified by clonidine, whereas plasma norepinephrine level was significantly reduced. Local skin erythema developed in three patients and dry mouth in six. These findings suggest that clonidine transdermal disks lower blood pressure in hypertensive patients, but produce local skin lesions and general side effects.