ALTHOUGH MAN'S ABILITY to perform exercise depends on many factors, it is normally limited by the capacity of the cardiovascular system to increase its transport of oxygen to active muscles. This increase is governed by complex changes in the circulation during exercise. How these changes are brought about has been summarized in excellent reviews by SjGstrand (r6g), Asmussen and Nielsen (6), and Rushmer and Smith (I 57). In the past decade, however, the increased application of cardiac catheterization in normal subjects has provided more complete information about circulatory dynamics in response to exercise than could be obtained earlier with indirect methods. Numerous studies have consolidated many of the earlier observations concerning basic regulatory mechanisms of the circulation and have reconciled apparent controversies. It is timely, therefore, to analyze the progress made since previous reviews?