1. Seventeen male patients with essential hypertension were studied after 4 weeks of placebo and after 8 weeks of beta-adrenoceptor-blockade therapy with atenolol (100 mg/kg). 2. The influence of the following factors on left ventricular wall thickness and left ventricular mass index as determined by echocardiography was examined: patient's age, duration of hypertension, arterial pressure, blood pressure variability, supine heart rate, maximal exercise heart rate, left ventricular wall stress and 24 h urinary catecholamines. 3. Left ventricular mass index was related to systolic blood pressure (r = 0.54, P less than 0.05) and to extent of increase in heart rate with maximal exercise (r = 0.62, P less than 0.05). No significant correlation was present between mass index and other variables. 4. After atenolol therapy, left ventricular mass index decreased by 14 g/m2 (12%). Changes in mass were related to its initial value (r = 0.69, P less than 0.01) and to % change in wall stress (r = 0.64, P less than 0.05). Patients who had a decrease in mass index of 10% or greater had an initially lower diastolic pressure (P less than 0.001). Other factors did not appear to influence significantly the regression of hypertensive left ventricular hypertrophy.