Four groups (25 males each) of Syrian golden hamsters were treated as follows: Group I was exposed to cigarette smoke after intratracheal instillation of 2 mg 7,12-dimethylbenz[a]anthracene (DMBA); group II was exposed to cigarette smoke without the initial dose of DMBA; group III received DMBA alone; and group IV (controls) received the vehicle alone. Hyperplastic and neoplastic lesions, mainly leukoplakic changes, developed in the upper respiratory tracts, especially in the larynx of group I hamsters, in a significantly higher incidence and earlier than in the group II anlmals. Other lesions in group I hamsters included squamous cell metaplasia of the nasal cavity, squamous cell papilloma of the oral cavity, and pharyngeal keratotic papillomas. The results demonstrated tumor-promoting and/or cocarcinogenic activity of cigarette smoke to the hamsters upper respiratory tracts.