Alterations in systolic time intervals during standardized cycle ergometer exercise testing were studied in nine normal middle-aged men. The results were related to oxygen uptake as well as cardiac output. Mild exercise (200 kpm min-1) led to no significant shortening of left ventricular ejection time (LVET), whereas the pre-ejection period (PEP) fell sharply (p<0.001). Heavier work rate was associated with a significant shortening of both intervals. The small variations of the normal values in the subjects studied suggest that this may be an appropriate measure of cardiovascular response to physiological stress. Comparison is diagnostically useful in patients whose heart rate response is within normal range. This is illustrated by a patient whose resting values were within normal limits, but exercise led to a greater decrease in LVET than in the normal group, possibly reflecting inability to increase the stroke volume to the expected level at this work rate. The use of systolic time intervals in association with exercise testing may be of use in revealing mild disturbance of myocardial function in patients who may show no evident abnormality at rest.