Synopsis A phenomenon of increased permeation as a function of time in the percutaneous absorption of urea and water led to this in vitro study with skin of hairless mice in a closed diffusion cell system. The Latin square method was used to assess the effect of four variables, namely temperature, time of hydration, concentration of urea and stirring of the donor-and receiver-cell phases on the physical structure of the skin and the permeability coefficients of urea and water. The scanning electron microscope and the transmission electron microscope were used to assess the physical changes in skin structure. Temperature and time of hydration, but not the concentration of urea or the stirring of the donor- and receiver-cell phases, affected the permeability of the skin to urea and water. The scanning electron microscope and transmission electron microscope micrographs confirmed these results.