The hemodynamic and renal responses to rapid intravascular volume expansion (VE) were studied in normal and areflexive dogs. "Isohemic" expansion was performed by infusing autologous blood (averaging 453 ml) thoroughly mixed with the circulating blood. In areflexive dogs cardiac output and arterial pressure doubled immediately after VE and fell back to control within 70 min; in normal dogs the circulatory response was less than one-third as great, but arterial pressure failed to return to control within 120 min. In the areflexive dogs, water, electrolyte, and total osmolar urinary output rose three- to fivefold after VE and declined thereafter, roughly following arterial pressure. In normal dogs the urine flow increased 40% immediately and rose further up to 70% in 60 min. No evidence for a natriuretic hormone was seen. It is concluded that mechanical factors are mainly responsible for the increased excretion of water and solutes after VE. Direct nervous reflexes to the kidney seemed to play a quantitatively minor role in the renal response.