In 5 normal persons and in 5 labile hypertensive patients, the hemodynamic response to exercise and its modification after beta-adrenergic blockade, was studied. In labile hypertensive patients, the changes in diastolic and mean blood pressure, left ventricular work and left ventricular stroke work were more pronounced than in normal subjects. After beta-adrenergic blockade, there was no longer a significant difference between the two groups. Moreover, the response of labile hypertensive patients after blockade was quite similar to the response of control group in normal conditions. The pharmacologic agent used Trasicor® did not limit the increase in cardiac output during exercise in the normal group. Its effect seems to be due mainly to a negative chronotropic action.