The in vitro passive transport of urea, mannitol, sucrose and raffinose across intact and ethanol treated human epidermal membrane was investigated. The intent of this study was to characterize the barrier properties and permeation pathways of these membranes for polar permeants under passive conditions. Based upon the relative permeabilities of these four solutes and hindered diffusion theory, the experimental data was adequately modeled for both membrane systems according to permeation through a porous membrane. Effective pore radii estimates for intact human epidermal membrane fell between 15 A to 25 A while similar estimates fell compactly between 15 A to 20 A for ethanol treated human epidermal membrane. Similarities between the relative permeabilities of human epidermal membrane for the four permeants studied and the relative permeabilities of these same permeants through ethanol pretreated human epidermal membrane indicate that significant similarities exist between the permeation pathways for both membrane systems. The results of this study have important implications for transdermal drug delivery in general and more specifically for strategies of designing effective chemical permeation enhancement systems.