The effects of hematocrit on renal hemodynamics and sodium excretion were studied in anesthetized dogs during both hydropenia and volume expansion. The hematocrit was decreased by isovolemic exchange with the animal's own previously harvested plasma and increased by isovolemic exchange with fresh, washed red blood cells. Renal perfusion pressure was maintained constant throughout the experiments by the adjustment of a suprarenal aortic clamp. During hydropenia, a decrease in hematocrit was associated with an increase in sodium and potassium excretion and solutefree water reabsorption. These changes were accompained by an increase in renal plasma flow and renal blood flow and a decrease in renal vascular resistance. Glomerular filtration rate was unchanged and filtration fraction was significantly decreased as hematocrit was lowered. Increasing hematocrit during hydropenia had the opposite effects on electrolyte excretion, solute-free water reabsorption, and renal hemodynamics. In another group of animals, hematocrit was lowered during volume expansion with either saline or plasma, then returned to the control level by isovolemic exchange with washed red blood cells. This increase in hematocrit during volume expansion had a similar effect on electrolyte excretion, solute-free water reabsorption, and renal hemodynamics as during hydropenia. These results therefore suggest that acute changes in hematocrit may significantly affect sodium excretion and renal hemodynamics during both hydropenia and volume expansion. The changes in solute-free water reabsorption and potassium excretion suggest that the alterations in hematocrit may affect primarily the reabsorption of sodium in the proximal tubule. The concommitant effects of hematocrit on renal vascular resistance and filtration fraction may mediate this change in sodium reabsorption by altering hydrostatic and oncotic pressures in the peritubular circulation.