The accumulation of intra-arterially injected I 131 albumin in the papilla of the kidney was measured in a series of 33 anesthetized dogs producing hypertonic urine. Although the data were widely scattered, an estimate of the average rate of this accumulation during the first half-minute following the start of the infusion was made. From the data, the plasma perfusion rate in the papilla under the experimental conditions was calculated and appears to average 25 ml./100 Gm./min.