1. Administration of renin to the rat causes an increase in the glomerular permeability to hemoglobin and a decrease in tubular absorption of this substance as measured by functional methods.
2. Morphologic examination in the rat and in the rabbit reveals an increased concentration of hemoglobin in the tubular lumina, confirmatory evidence of increased permeability. However, there is no significant effect upon the formation of hemoglobin droplets within the renal cells.
3. By analogy with the events observed in neutral red "secretion," it is suggested that hemoglobin is absorbed by both a "direct" and an "indirect" mechanism and that renin administration affects or depresses the "direct" mechanism only.