In vitro percutaneous absorption studies were carried out for eight chemicals using full thickness rat skin and human stratum corneum. The purpose of the studies was to compare the rates of absorption for the two species. For each of the chemicals, the observed rate using full thickness rat skin was greater than that observed for human stratum corneum. The ratios of the rates (rat/human) varied from 1.7 to 5.8 with a mean value of 3.1. The chemicals tested were tritiated water, 2-ethoxyethyl acetate, diethylene glycol monobutyl ether, urea, di(2-ethylhexyl) phthalate, 2-ethylhexanol, ethyl 3-ethoxypropionate, and 2-propoxyethanol. The chemicals were chosen to represent a wide range of physical properties and permeability constant values. It was concluded that rat skin was more permeable than human skin for each of these eight chemicals. This conclusion is supported by similar findings from studies in other laboratories and suggests that results from studies in the rat overestimate skin absorption in man.