Previous studies in this laboratory have shown that the combined exposure of hydrogen chloride (HCI) and formaldehyde vapors (HCHO) elicited a significant incidence of nasal cancer in rats. In studies performed elsewhere, it has been demonstrated that exposure to formaldehyde alone induced a high nasal cancer response in rats. We wished to determine whether concurrent exposure of hydrogen chloride would enhance the tumorigenic effects of formaldehyde. Two exposure techniques were used. In one hydrogen chloride and formaldehyde were premixed at high concentrations before entry into the exposure chambers in order to maximize the formation of reactive alkylating agents. In the second the hydrogen chloride and formaldehyde were introduced separately into the exposure chamber. Appropriate control exposures consisting of formaldehyde alone or hydrogen chloride alone or air alone were also performed. The results show that nasal cancer incidences were induced in all animals receiving HCHO regardless of concurrent exposure to hydrogen chloride. The tumors were predominantly squamous cell type arising from the anterior portion of the nasal cavity. This study demonstrates that hydrogen chloride does not appreciably influence the nasal carcinogenicity of formaldehyde.