Cofactors involved in respiratory tract carcinogenesis were studied in Syrian golden hamsters or in rats using benzo(a)pyrene as the carcinogenic agent. These factors included severe tissue damage induced by electro-coagulation, glass fibers administered by intratracheal instillation, acetaldehyde as irritant vapor, food restriction, and nutrients such as vitamin A and saturated and unsaturated fats. In addition, the effects of a combined exposure to four different major gaseous cigarette smoke components--methyl nitrate, isoprene, methyl chloride and acetaldehyde--and to one solid cigarette smoke component--norharman--were examined in short- and long-term inhalation studies. An interesting finding was the carcinogenicity of acetaldehyde, of which the possible mechanism is briefly discussed. Another conspicuous observation was the substantial increase in number and size of lipid droplets in alveolar fibroblasts of hamsters fed a high vitamin A diet.