Left ventricular ejection fraction (LVEF) was measured at rest and during supine bicycle exercise in 31 men with arteriographically defined coronary disease and in 15 normal men. LVEF was calculated from a left ventricular time vs activity curve (collimated scintillation probe, 99m Technetium) as the fracitonal fall in count-rate divided by the background-corrected left ventricular end-diastolic count-rate. In normal men LVEF at rest averaged .59 +/- .06 (+/-SD) and during exercise was .72 +/- .08. LVEF did not increase with exercise in men with coronary disease (.55 +/- .03 to .57 +/- .03; N = 31; AVE +/-SEM; NS). In 17 men with coronary disease who had ST segment depression with exercise, LVEF either decreased or was unaltered in all (55 +/- .04 to .49 +/- .03; P less than 0.05); whereas in 14 without ST depression, LVEF increased in 10 (71 per cent) and was unaltered in 4 (29 per cent) (.54 +/- .04 to .66 +/- .04; P less than 0.01). Results suggest that LVEF during exercise normally increases, but in men with coronary disease LVEF either fails to increase or actually decreases. In addition there appears to be a relationship between ST segment changes during exercise and ejection fraction.