Abstract. Thirteen men with untreated essential hypertension in WHO stage I, all working, have been studied ambulatorily. Oxygen consumption, heart rate, cardiac output (Cardiogreen) and intraarterial brachial pressure were recorded at rest in supine and sitting position and during steady state work at 300, 600 and 900 kpm/min. The subjects were treated with Clonidine as the sole drug for one year and the hemodynamic study was repeated. The reduction in the intraarterial pressure was usually due to a decrease in cardiac output which again was due to a reduction in heart rate. The calculated total peripheral resistance did not change significantly. The pressure reduction was greatest at rest. During submaximal exercise the differences between the hemodynamic parameters before and after therapy were small.