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RADIOISOTOPE TECHNIQUES IN THE STUDY OF PERCUTANEOUS ABSORPTION* By FREDERmK D. MAI•K•NSON, M.D. Section of Dermatology, Department of Medicine, University of Chicago, Chicago, oe/linois WHILE THE SUBJECT ofpercutaneous absorption (1, 2) has been widely investigated in the past, our knowledge in this field is still rather limited. Consequently, it is most encouraging that the recent introduction of radio- isotopic techniques now holds promise of providing accurate and definitive conclusions from almost any experimental problem in this field. Previously the methods most widely used to ascertain cutaneous absorption of a given substance were chemical determinations in skin, blood, urine, various internal organs or expired air elicitation of certain local or systemic allergic, biological or toxic actions histochemical techniques and study of concentration changes of substances in solution following a. pplication to the skin. In the near future it is likely that these various procedures will be largely superseded by radioisotopic techniques which present numerous distinct advantages. These include radioautographic demonstration of rate and depth of penetration into the skin simplified qualitative and quantitative absorption studies by measurement of radioactivity in blood, urine, expired air or internal organs quantitative studies in the skin itself by such methods as radioautographic densitometry or grain-counting and precise measurements of absorption of substances normally found in body tissues but otherwise indistinguishable from them by older qualitative techniques. Before discussing the more significant results of percutaneous absorption studies with radioisotopes, the salient factors relating to cutaneous absorption will be reviewed. Passage of materials into the skin occurs through two main routes, the transepidermal and the transappendageal, the latter implying absorption primarily through the pilosebaceous apparatus. Transepidermal absorp- tion, particularly as regards water, electrolytes and perhaps certain water- soluble nondissociable substances, is determined largely by a barrier superficially situated between the stratum comeurn and the Malpighian * Presented at the September 15-16, 1955, Seminar, New York City. 109