Changes in ventricular function caused by activities of daily living, including standing, walking, stair climbing, and mental stress, were evaluated using a radionuclide device that recorded left ventricular function on a beat-by-beat basis. The ambulatory monitor was positioned over the patient's left ventricle after a gated blood pool scan. Monitoring revealed a 10% increase of left ventricular ejection fraction from baseline to brisk walking, an 18% increase during stair climbing, and a 6% increase with mental stress. In some subjects, however, the increase in ejection fraction during mental stress exceeded that during exercise.