Radio-active plastic microspheres with an average diameter of 15 mu were used in eight dogs to measure renal perfusion and the results compared with time-activity curves before and after constriction of the left renal artery. It was shown that the values obtained for renal blood flow by functional scintigraphy are not accurate, particularly for moderate or low blood flow. The reason for this is primarily the distortion of the intravenously injected radio-active bolus and the consequent deviation from the so-called delta function.