The distribution of the radioactivity following an interaarterial injection of (-131-labeled macroalbumin (MA) was observed in 7 human kidneys and compared with the renal distribution of MA and microspheres in the dog. In the renal cortex the radioactivity per gram of dry tissue in the juxtamedullar and middle cortex was 20.5 and 44.2 per cent of that observed in the outer cortex. The relative radioactivity in the juxtamedullary cortex was significantly (P is less than 0.01) greater in the human than in the dog kidney. In the renal columns the radioactivity was similar to that observed in the middle and outer cortex with incresing values towards the renal hilus. Only traces of radioactivity were observed in the renal medulla. The findings indicate that the distribution of the renal cortical blood flow is similar in the human and the dog kidney with the exception of a greater relative blood flow in the juxtamedullary part of the human kidney.