The effect of clonidine (average 0.24 mg/day) and propranolol (average 105 mg/day) on home blood pressure readings in 16 patients with borderline hypertension was investigated in a randomized, double-blind, placebo crossover design. Patients could detect small but significant decreases of blood pressure with both active compounds (-8/-5 with propranolol and -11/-7 with clonidine). The larger mean blood pressure decrease from clonidine vs propranolol was significant (p less than 0.015). Small doses of sympatholytic agents might control the blood pressure in patients with borderline hypertension, and the home blood pressure technique is a convenient tool to detect and monitor such changes. Biochemical predictors of the responsiveness to clonidine were investigated. There was no difference in placebo norepinephrine and renin values between better and lesser responders to clonidine. Plasma norepinephrine fell with clonidine treatment, but with no relationship to the blood pressure response. Plasma norepinephrine response to clonidine might reflect not only the central withdrawal of sympathetic tone, but also, in part, the effect of clonidine on peripheral presynaptic alpha 2-receptors.