The hemodynamic effects of reducing venous return were assessed beat by beat at the onset of upright dynamic exercise. Mean arterial pressure (MAP), heart rate, and left ventricular end-systolic (ESV) and end-diastolic volumes (EDV; two-dimensional echocardiography) were measured in 10 healthy men during 20-s trials of upright cycling (30 W; 60 rpm). Exercise was performed either with or without venous occlusion of the legs (bilateral thigh cuffs inflated to 100 mmHg) in a random order. Without venous occlusion, MAP and cardiac output (CO) increased, and total peripheral resistance (TPR) decreased (P < 0.05) during the first approximately 10 beats after the onset of exercise. Initially, the CO response was accounted for by a rapid heart rate acceleration and, after approximately 15 cardiac cycles, by an increase in stroke volume, which occurred with a decrease in ESV and no change in EDV. With venous occlusion, EDV decreased and stroke volume did not rise during exercise. Thus the CO response was blunted by venous occlusion and MAP did not increase initially. However, after approximately 13 heart beats, MAP increased with no change in TPR. These findings suggest that compensatory mechanisms can elicit an increase in MAP at the onset of mild upright cycling when the CO response is blunted by reducing venous return.