This report assessed in rats the carcinogenicity of inhaled 1-nitropyrene, a compound frequently adsorbed to diesel particulate matter, and whether this effect is modified when 1-nitropyrene is associated with particles or irritant gases. Dr. Wolff and colleagues exposed rats to atmospheres containing 14C radiolabeled 1-nitropyrene alone or in combination with gallium oxide, sulfur dioxide, or both. After exposure, tissue samples were analyzed for radiolabel content to determine the tissue distribution of 1-nitropyrene and its metabolites.