Synopsis-Four parameters for assessing the interaction between human STRATUM COR-

NEUM and WATER are described. Two methodologies, ELASTIC MODULUS and STRESS RELAXATION, for determining the MECHANICAL PROPERTIES of stratum corneum have been utilized. It is shown that both of these parameters depend on the moisture content of the stratum corneum, i.e., the ambient relative humidity. The rate of WATER VAPOR ABSORPTION by human stratum corncure, the third parameter exam-ined, is a function of the ambient relative htm•idity. Surprisingly, the equilibrium moisture content of stratum corneum at humidities below approximately 80 % appears to be essen-tially the same for unextracted stratum corncure and for stratum corncure extracted with lipid solvents. The fourth parameter, the rate of WATER VAPOR TRANSMISSION through stratum corncure in vitro, is a linear ftmction of the ambient relative humidity and has been shown to be markedly affected by changes in temperature. INTHODUCTION Since Blank's observation in 1952 (1), that the water content of skin is re-sponsible for its softness, the properties of epidermis as a function of its mois-ture content and of the presence of "natural " and extraneous moisturizers have been studied extensively by cosmetic hemists and dermatologists. The subject of transepidermal moisture loss in vivo under various ambient conditions of humidity, temperature, and air flow has been studied by several investigators. Of particular interest is a paper by Grice et al., who indicated that the relationship between transepidermal water loss and ambient relative humidity in vivo is not linear (2). This is not in accord with the data by Wild-nauer et al., who suggested that the relationship between mechanical proper-ties and relative humidity is linear (3)