The DNA damage associated with benzo(a)pyrene (B[a]P) and formaldehyde (HCHO) exposure in rat tracheal implants was determined by alkaline filter elution adapted to measure DNA-protein cross-links (DPC) in vivo. In addition, histopathological responses of the tracheal epithelium were quantitated after multiple exposures to 20 μg B[a]P and 0.2% HCHO. Compared to either agent alone, combined exposure for 1–4 weeks caused an increase in cellular atypia and greater thickness of hyperplastic and metaplastic lesions. HCHO exposure resulted in a dose-dependent increase in DPC with a maximal response of 85% DNA filter retention at 0.2% HCHO, which were mostly removed by 72 h. B[a]P did not cause DPC, but when tracheas were pre-exposed to 20 μg B[a]P followed by 0.05% HCHO there was a 15% decrease in HCHO-induced DPC. This competition between B[a]P and HCHO for sites presumably on DNA does not offer a clear explanation for their markedly enhanced co-carcinogenicity observed in previous studies, but does demonstrate the interaction between the two agents in tracheal epithelium.