A canine model of squamous cell lung cancer has been developed through studies with 110 dogs exposed by 11 focal endobronchial regimens to chemical carcinogens: benzo(a)pyrene, nitrosomethylurea, methylcholanthrene, and dimethylbenzanthracene. A combination of nitrosomethylurea and benzo(a)pyrene caused the first invasive cancer after 5.5 years. Toxic side-effects resulted from either nitrosomethylurea or high-dose dimethylbenzanthracene given by bronchial submucosal injection and from adjuvant immunosuppression with azathioprine and corticosteroids. Four regimens in 58 dogs caused 31 cancers, including five T1-2 N0 M0 cancers, 17 metastasizing carcinomas, and nine carcinomas of lesser stages. The following regimens caused cancers: sequential benzo(a)pyrene, nitrosomethylurea, and yttrium 91; benzo(a)pyrene and topical nitrosomethylurea; low-dose dimethylbenzanthracene; high-dose methylcholanthrene. The most suitable regimen to date has been 30 mg of methylcholanthrene given by submucosal injection every 2 to 3 weeks; this produced cancers at preselected sites within 2 years of first exposure in eight of 10 dogs. The neoplastic continuum has followed a predictable, reproducible sequence that regularly began with epithelial hyperplasia. Squamous metaplasia occurred in 6 to 18 weeks; it was followed by progressive squamous atypia. The interval until invasive cancer developed varied with the regimen employed; it was about 20 months with methylcholanthrene. Serial cytologic specimens, studied by image analysis, revealed progressive increase in mean total cellular deoxyribonucleic acid content from diploid in normal cells to greater than tetraploid in cancer cells (p less than 0.01). We have recently been successful with serial passage of four canine lung cancers from four to twelve transplant generations in nude mice. There is now a predictable large animal model of squamous cell lung carcinoma at preselected site(s) that closely resembles human lung cancer. The preneoplastic period is short enough to be fiscally defensible, but long enough to permit study of the biologic changes during endobronchial carcinogenesis.