The response of the heart rate and arterial pressure to a standard Valsalva maneuver was studied in 36 patients with atrial septal defect and compared with that in 50 normal subjects. A quantitative method for describing the response was devised. The normal response was rather variable, but all normal subjects showed some narrowing of pulse pressure during the strain and some systolic pressure overshoot in the period after release. In every normal subject the narrowing of the pulse pressure during the strain or the systolic pressure overshoot was a change of at least 15 per cent from the control value. The changes in pressure were accompanied by tachycardia during the strain and bradycardia during the overshoot period, and the strain tachycardia or the overshoot bradycardia was at least 10 per cent altered from the control value in all normal subjects. By these criteria, 31 of 36 patients (86 per cent) with atrial septal defect showed abnormal responses, and in 27 of 36 (75 per cent) this was evident in the heart rate response, so that this test could be used to some advantage as a simple screening test of outpatients with the electrocardiogram. The abnormal response is presumed to result from the increased intrathoracic blood volume, allowing adequate cardiac output to continue despite temporary obstruction of venous return, rather than from myocardial failure, which was not present in these patients.