N′-Nitrosonornicotine (NNN) and N′-nitrosoanabasine (NAB) were synthesized and injected subcutaneously three times weekly in male and female Syrian golden hamsters for 25 weeks (total dose 375 mg). N-Nitrosopiperidine (NPD) served as positive control (total dose 150 mg). Within 83 weeks 12 out of 19 hamsters given NNN had developed tracheal tumors, 1 had a carcinoma of the nasal cavity. During the same time none of the hamsters given NAB developed tumors, whereas all 20 hamsters given NPD developed tracheal tumors and about 50% had additional neoplasms in the nasal cavity. Under the described conditions and in comparison to NPD, NNN appears to be a moderately active tumorigenic agent in the upper respiratory tract of Syrian hamsters, whereas NAB is inactive.