For a detailed analysis of the water barrier function of skin, it is necessary to obtain effective physical parameters related to the rate of transepidermal water loss (TEWL). By using a modified ventilated chamber measurement system, with high sensitivity and response, we have determined the apparent mass transfer coefficient of water, K, for the stratum corneum (SC) and the water vapor pressure, P-d, at the interface between SC and underlying living cells of epidermis from analyses of water distribution in the contact zone of carrier gas and skin surface. By data fit optimization of the calculated water flux to the measured one, the in vivo K value for human skin was determined to be (0.30 +/- 0.14) x 10(-8) kg/(m(2.)s(.)Pa) (n = 4, mean +/- SD) which is close to the corresponding in vitro value of (0.12 +/- 0.02) x 10(-8) kg/(m(2.)s(.)Pa) (n = 6, mean +/- SD) for porcine SC membranes. It is, therefore, concluded that the apparent mass transfer coefficient of water for SC can be estimated in vivo and noninvasively by the method developed in this study and will lead to a more precise and reliable evaluation of skin barrier function than the rate of TEWL itself.