In captive Harris' sparrows (Zonotrichia querula), plasma and blood volume increased significantly during stages 2 and 3 of postnuptial molt, when extensive feather replacement was taking place in the wings, tail and body tracts. Erythrocyte number did not increase significantly until molt was essentially complete. Consequently the 6% mean maximum reduction in hematocrit that was observed during molt appeared to be largely the result of plasma volume expansion. This plasma volume expansion was not due to changes in the osmolality or concentration of proteins within the blood plasma since both decreased significantly during intense molt.