Radiological heart volume estimations and echocardiographic studies were performed on 30 male and 15 female middle distance athletes at rest, during supine ergometric exercise at 10 and 100 Watt and 1 minute after excerise. During exercise, there is an increase in cardiac output, in the small hearts of 6%, in the large hearts of 16%, due to a small enddiastolic increase and an associated decrease in the endsystolic left ventricular diameter. At rest and at 50 Watt, the cardiac output, corrected for body surface area, was practically the same for all volunteers, at 100 Watt the volunteers with the largest hearts had the highest cardiac output. This is interpreted as evidence for increased cardio-circulatory reserves. The contractility parameters at the various exercise grades varied dependant on heart size; when related to heart rate the variations were practically abolished. The results suggest that variations in the dynamics of small and large hearts are not primarily due to intrinsic factors but rather related to extrinsic factors such as sympathetic drive expressed by the changes in heart rate.