SUMMARYA method is presented for simultaneous in vitro growth and measurement of permeability of human skin. During culture of specimens, the parakeratotic layer formed is shown to possess considerable barrier function. Maximum barrier capacity of this layer in vitro is attained at 14–17 days’ incubation. When the stratum corneum is removed prior to incubation, the maximum barrier capacity is reached in 3–4 days. The difference in behaviour between intact and stripped skin is reflected in the pattern of DNA synthesis of the basal cells, which show an enhanced proliferation after stripping. Barrier regeneration of stripped skin is seen closely to resemble the parallel situation in