Direct arterial pressure and electrocardiogram have been measured continuously over a period of 24 hours in 15 patients. Observations have been made on the behaviour of these variables during 30 separate episodes of motor car driving. The patients were divided into three groups: (1) five normotensive subjects, (2) five patients with essential hypertension who were not receiving therapy, and (3) five patients with angina pectoris who were either normotensive or hypertensive.In all but one subject, apart from variable changes in heart rate, no significant arrhythmias or S-T segment changes were observed in the electrocardiogram. The arterial pressure remained remarkably stable throughout the driving period in all three groups and there was no significant difference between the levels of blood pressure at the beginning and end of a journey. There were short periods of raised arterial pressure during driving related to such episodes as overtaking, but these quickly returned to baseline levels. Two patients experienced anginal pain during driving.It is concluded that motor car driving does not have such a pronounced effect on the blood pressure as might have been thought.