Studies of experimental tobacco smoke carcinogenesis have suffered from the lack of a conveniently available and well-characterized device for exposing animals to tobacco smoke for inhalation. The Walton Horizontal Smoking Machine, a commercially available system designed to expose up to 20 mice to the smoke of a single cigarette, may fulfill this need. This system produced a uniform smoke aerosol of predictable concentration and appropriate composition for cigarettes with high delivery of nicotine (40 mg total particulate matter, 2.6 mg nicotine, and 17 cm3 carbon monoxide per cigarette) and with low delivery of nicotine (30 mg total particulate matter, 0.3 mg nicotine, and 17 cm3 carbon monoxide). In this experiment C57BL and DBA/2Bd strains of mice were used. Limitations of the concept of exposing animals to standing smoke were defined.