In eight dogs anesthetized with chloralose and urethan, Valsalva-like maneuvers (VLMs) were performed and central blood volume (CBV) measured following incremental volume loading with blood-dextran. Changes in blood pressure during VLM were characterized by 1) maximum change in systolic pressure (deltaPmax) during VLM; 2) change in systolic pressure during the first five beats of VLM (deltaP1-5); and 3) the change in pulse pressure during the first 10 beats of VLM. In three dogs, we also measured changes in right and left atrial pressure (PRA, PLA), esophageal pressure (PE), and pulmonary and aortic blood flow. We found that volume loading produced a continuous spectrum of hemodynamic changes culminating in the "square-wave" response. deltaPmax and deltaP1-5 correlated with changes in CBV. Changes in hemodynamic responses were produced by increases in CBV, and by incomplete prevention of systemic venous return during VLM. The mechanism of the square-wave response may be a combination of both these effects, with the latter the more significant of the two.