Advantage is taken of the fact that the absorption of light by a transilluminated tissue varies with its blood content to study the skin circulation with the aid of a photoelectric plethysmograph which is described. The sources of error are discussed. The essential validity of the photoelectric plethysmogram is demonstrated in simultaneous plethysmo- grams, photoelectrically and mechanically recorded, of the fingers, in instances where the vascular responses to some common procedures were followed. The argument is advanced and supported by suitable data that under resting conditions, with normal circulatory dynamics, the vol. pulse of the skin area is a measure of the richness of the arterial blood supply of that area. The skin areas thus studied arrange themselves in descending order of the richness of their arterial supply thus: finger pad, ear lobe, toe pad, palm of hand, skin of forehead and face, dorsum of finger, of hand and of foot, forearm, knee and tibia. The effect of climate on the arterial blood supply of these areas is also indicated.