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The implications of Starling's law of the heart, as stated in the Linacre Lecture (1), miiake it clear that physical exercise brings about increased venous return and increased diastolic filling of the heart. This, in terms of the Starling concept, induces sufficient increase in the energy of contraction to drive increased volumes of blood into the aorta with each stroke. While Starling was aware that "the heart is subject to the control of the central nervous system by means of its inhibitor and augmentor nerves . . ." he undoubtedly placed the emphasis, insofar as cardiac adaptation to stress is concerned, on intrinsic response of the miiyocardium to change in end-diastolic fiber length. It follows, if Starling's Linacre Lecture is credited, that the heart must meet the demiiands imiiposed by physical exertion, in part at least, by increasing its stroke volume.This was indeed found to be the case in older studies on the effect of exercise on human circulatory function (2-4) ; these studies served further to establish the view that increase in both stroke volume and pulse rate are utilized by the organism during exercise.A different view has been gradually formulated by Rushmer who, using continuous measurements of internal left ventricular diameters (5) and circumference (6) in unanesthetized dogs exercising on a treadmill, was not convinced that response to such stress was entirely in accord with Starling's law. These results, and an examination of other workers' data on human subjects, ultimately led him to a radical revision of the older view concerning the interdependence of increase in stroke volume, pulse rate and arteriovenous oxygen difference in adaptation to exercise. In his words (7) Because data on human subjects from our laboratory were in conflict with Rushmer's conclusions, it was decided to re-examine our results anid to add new studies to fill in the gap between the resting state and very heavy exercise loads. MET11ODStudies on 26 normal men, aged 19 to 63 (average 28) form the basis for this report. Data on 15 of the subjects were previously reported in studies on maximal oxygen intake (9, 10). Since the previous experiments were designed to yield information oinly under resting conditions (subject stainding) and at exercise loads producing maximal oxygen intake, 11 additional subj ects were tested under different conditionis. In 5 subj ects, initial measurements were made with the subj ect supine and at rest. Immediately following, the measurements wvere repeated with the subj ect standing at rest.Further measurements were then made at very low levels of exercise. In the other 6 subjects the procedure was the same except that Ino studies were done with the subj ect supine.The experimental procedure was the same as that previously reported (9). Brachial venous and arterial catheters were inserted into the left arm. For measurement of cardiac output, 10 mg of Evans blue (T-1824) was delivered at the end of the catheter into the brachial vein with the subject at rest or, in the exercise stud...

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