Topical application of N-methyl-N-benzylnitrosamine [(MBN) CAS: 937-40-6] to Syrian hamster buccal pouch epithelia resulted within 4 days in the induction of epithelial cell foci with histochemical gamma-glutamyl transpeptidase (GGTase) activity. Approximately 94% of the foci consisted of individual cells or nests of 8 cells or less, as visualized in wholemounts of GGTase-stained pouch epithelia. During a regimen of twice weekly MBN applications, the GGTase-stained foci increased in size to a maximum diameter of approximately 0.4 mm by the third week. In histologic sections, the enlarging foci were similar morphologically to those of the adjacent GGTase-negative epithelium. Groups of animals treated with MBN for 2, 4, and 8 weeks developed a buccal pouch tumor incidence of 43, 86, and 100%, respectively, within 39 weeks or less. Of the 138 papillomas and carcinomas examined, 11 contained patchy GGTase activity.