AbstractTritium‐labelled benzo[a]pyrene ([3H]BaP) was applied to mouse skin in acetone or mineral oils of differing viscosity. Epidermal DNA and protein were extracted after 24 or 48 h and the degree of adduct formation determined by the radioactivity present.When [3H]BaP was applied in acetone, the degree of DNA and protein binding was around 15–20 times greater than that observed when a low‐viscosity oil was used as a vehicle. When applied in oils of differing viscosity, however, only a twofold difference was seen across the whole viscosity range (13.5 cStSt = stokes, a unit of kinematic viscosity (10−4m2s−1)at 40°C to 1665 cSt at 60°C). From measurements made of urine and fecal radioactivity and from small‐scale investigations using other routes of administration, it was clear that the grooming activity of the animals had a marked effect on skin absorption and macromolecular binding. It is possible that greater grooming activity with low‐viscosity oils may explain why oil viscosity did not have a greater effect on binding levels, but further studies are needed to investigate this.These findings may have important implications in the interpretation of long‐term skin painting studies and may assist in the interpretation of analytical data and short‐ter