Autoregulation of renal blood flow (RBF) was studied in rats that underwent equivalent blood volume expansion with saline (Sal; 5% body wt), 7% BSA solution (1.4% body wt), and reconstituted whole blood from donor rats (WBL; 1.4% body wt). Renal perfusion pressure (RPP) and renal neural reflexes were prevented by clamping RPP and sectioning the vagus, baro/chemoreceptor, and renal nerves. Sal and BSA expansion increased RBF by approximately 60%, whereas no effect was observed with WBL. RBF autoregulation was markedly attenuated after expansion with cell-free solutions, but no change occurred in WBL-expanded rats. Correction of the fall in hematocrit in Sal- and BSA-expanded rats restored RBF and its autoregulation to control levels. Expansion with Sal or BSA after inhibition of renal vascular tone with intrarenal infusion of papaverine still increased RBF and further changed the RBF-RPP relationship. These findings suggest that the hemodilution plays a central role in the reduction of renal vascular resistance and in the attenuation of the autoregulatory efficiency of renal circulation that accompany expansion with cell-free solutions.277R1662-74