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SUMMARY.— The ability of a number of liquids to increase the permeability of human skin in vitro has been assessed in terms of their power to accelerate the percutaneous penetration of tri-n-propyl phosphate (TPP).

The most effective “accelerants”. 8 M-urea and dimethylsulphoxide (DMSO), increased the permeability of full thickness skin to TPP up to 190 times, they were also amongst those compounds, which produced the greatest reduction in skin impedance and the most swelling of the stratum corneum, suggesting that part of their effectiveness may be due to an ability to lower the diffusional resistance of the stratum corneum.

The acceleratns were all able to extract soluble components from the stratum corneum; DMSO extracted lipoprotein, and chloroform-methanol extracted phospholipids, suggesting the possibility of ultrastructural modifications consistent with an increase in permeability.

For a liquid to be a good accelerant it must also release the penetrant readily to the aquesous milieu of the viable epidermis. This process could be hindered by an excessively unfavourable partition coefficient or by the extremely low water solubility of a penetrant.