The optimal pressure in the renal artery during initial perfusion with Collins' solution by spontaneous pressure in canine kidneys is 40-50 cm H2O and in human kidneys 40-60 cm H2O. An average-sized human kidney can be perfused by a full current which is limited only by the lumen of the infusion set. In smaller than average-sized human kidneys the current must be reduced to diminish the pressure in the renal artery to the optimal value. In prolonged hypothermal perfusion by means of a Gambro apparatus it will be necessary to assess the optimal pressure by direct measurement in the renal artery in intact average-sized human kidneys because the pressure reading on the manometer is not identical with the optimal pressure in the renal artery.