Hemoglobin infused into rats was excreted mainly by the kidneys. The effects of hemoglobin on renal tubular functions were investigated in this study. During intravenous infusion of 6% stroma-free hemoglobin (SFHB) solution, there was a 30% reduction in glomerular filtration rate (GFR). The reduction in GFR was directly related to the urine hemoglobin concentration. The rate of urinary sodium excretion (8.2 mumol/min) was significantly higher than the control rate (2.8 mumol/min), suggesting proximal tubular damage in the presence of intratubular hemoglobin. This was further substantiated by the demonstration of a 10-fold increase in urinary N-acetyl-beta-D-glucosaminidase (NAG) activity during SFHB infusion. In contrast, dextran-hemoglobin (DXHB) infused into rats was not excreted by the kidneys. During the infusion of 6% stroma-free DXHB solution, there was no significant change in the GFR. Urinary sodium excretion was lower than the control value whereas urinary NAG activity was slightly elevated. The observations suggest that the presence of hemoglobin in the renal tubules gives rise to impairment of tissue function and structure. DXHB, on account of its larger molecular size, does not enter into the renal tubules, and therefore does not cause such impairment.