This study was undertaken to evaluate the effects of three vasoactive drugs—levarterenol (norepinephrine) bitartrate, isoproterenol (Isuprel) hydrochloride, and phenoxybenzamine (Dibenzyline)—upon renal blood flow using a recently developed technique for measuring local tissue blood flow. The technique, based upon the detection of hydrogen with a platinum electrode1 implanted in the tissue of interest, has been shown to quantitatively measure blood flow in its vicinity as the respired gas is washed out of the tissue and cleared by the lungs. This method has the advantage over other means of determining blood flow in that operative trauma is minimized and blood flow to the functioning parenchyma (rather than average flow through the entire organ) is measured. The effects of various drugs on flow, resistance, and survival have been studied by many investigators. The studies on flow in the heart and kidney may be classified as normovolemic and hypovolemic. Morris,2 Onnis,