The effects of acebutolol (800 mg per day for three days) on pulse rate, blood pressure, plasma noradrenaline and plasma renin activity were measured in nine patients with essential hypertension, at rest in a horizontal position, after five minutes standing upright and after walking for one hour. Acebutolol reduced pulse rate, blood pressure and plasma renin activity in all three positions. Treatment has no influence on plasma noradrenaline in a horizontal position, but significantly reduced its orthostatic stimulation. The inhibition of noradrenergic response to an orthosatic position is compatible with a central or presynaptic effect of acebutolol. This effect would not appear to be essential to its antihypertensive action since acebutolol reduced blood pressure but not plasma noradrenaline in a horizontal position. Blood pressure response to treatment was positively correlated to initial values of plasma renin activity measured after walking for one hour and plasma noradrenaline measured after five minutes in an orthostatic position. On the basis of this correlation, measurement of these two hormonal indices is of predictive value, within the limits of a short period of treatment with acebutolol and strictly defined hospital conditions.