Diffusion tubes were used to measure the degree of in vitro penetration of tetrabutyl lead, lead naphthanate, lead nuolate, lead acetate and lead oxide in excised guinea pig skin and human skin from autopsy. Tetrabutyl lead demonstrated the greatest penetration in skin from both guinea pig and man. Lead nuolate, lead naphthanate and lead acetate followed in descending order in the human tissue. A similar pattern occurred with guinea pig skin in most cases. There were no measurable amounts of lead oxide absorbed in either species. In vivo absorption was measured by applying 300 mg/kg tetrabutyl lead, lead nuolate, lead naphthanate or lead oxide to the shaved backs of guinea pigs for 7 d under occluded wrappings. Tetrabutyl lead was present in tissues in the highest quantities. Lead nuolate was present in greater amounts than lead naphthanate in the liver and kidneys. Lead acetate was the most poorly absorbed with the exception of lead oxide which demonstrated no absorption.