SKIN PENETRATION 497 phosphate have shown the similarity of results using excised and intact skin. Bettley and Donoghue (48) have used excised skin for several days without noticeable change in the results. Where full thickness skin is used the penetrant has to cross the dermis, whereas in intact skin it would probably be picked up by the capillary network at the dermo-epidermal junction. For aqueous solutes the differ- ence would probably be small, whereas for sparingly water soluble pene- trants it may be significant. The use of epidermis alone has the disadvantage that it is fragile and holes appear at the hair follicles. In practice, however, this technique is relatively simple, will give rapid and reproducible results and allows direct measurement of the amount of penetrant diffusing, the rate of diffusion and the amount retained by the skin. CONCLUSIONS With the ever increasing production of new synthetic chemicals and their widespread use in our daily lives, the need to ensure that they are not harmful to the skin, or by penetrating the skin, is becoming more necessary. Although a considerable volume of work has been carried out to eluci- date skin structure, physiology, barrier properties and the mechanisms by which substances enter and cross the skin, there is clearly still much to learn. It is necessary to elucidate, eventually at the molecular level, how the skin keeps many invading chemicals at bay. For those substances that do penetrate the skin, those physico-chemical properties, or combination of properties, which enable them to breach the skin barrier must be worked out. With this information it might then be possible to spot a potential skin penetrant at an early stage. In particular, information on the influence of vehicles on skin penetration is still sparse and often contradictory. There is much scope for developing vehicles which will enable the drug to reach the site of action rapidly and which will maintain a sufficient concentration at the site for the required length of time. (Received: 9th September 1•68) REFERENCES (1) Schwenkenbecher, A. Arch. Anat. _Physiol. 121-165. (1904). (2) Mallkinson, F. D. and Rothman, S. Handbuch der Haut und Geschlechtshranhheiten. in J. Jadassohn. (1963). (Springer, Berlin).

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