Syrian golden hamsters were exposed to 0 or 250/400 ppm furfural vapour, 7 h/day, 5 days/week for a period of 52 weeks. Simultaneously, a proportion of the animals were given either intratracheal inslillations of benzo [alpha] pyrene (BP) or subcutaneous injections of diethylnitrosamine (DENA). All treatments were stopped after 52 weeks. The study was terminated after 81 weeks. Furfural exposure caused yellowish discolouration of the animals' coat, irritation of the nasal mucosa, growth retardation, atrophy and downward growth of sensory cells of the olfactory epithelium, degenerative changes in Bowman's glands, and the occurrence of cyst-like structures in the lamina propria beneath the olfactory epithelium. There was no evidence of furfural possessing carcinogenic activity. In a previous instillation experiment with furfural, slight indications had been obtained of furfural potentiating the carcinogenic effect of BP, but in the present study the carcinogenic effect of BP or DENA on the respiratory tract did not appear to be influenced by furfural exposure. It was concluded that little if any, significance can be attached to furfural as a co-factor in respiratory tract carcinogenesis.