In assays of Areca-specific N-nitrosamines, 3-(methylnitrosamino)propionaldehyde (MNPA) exhibits higher cytotoxicity than nitrosoguvacine (NGC), nitrosoguvacoline (NG) and 3-(methylnitrosamino)propionitrile (MNPN). NGC is not mutagenic. However, NG is a weak carcinogen in F344 rats while MNPN is a potent carcinogen; MNPA had thus far not been tested. In this study MMPA was injected s.c. at a dose of 6.57 mg three times weekly for 15 weeks (total dose 2.6 mmol/rat). During the 100 weeks of the bioassay, the treated K344 rats, and especially the females, showed significantly less weight gain than the control animals, indicating high toxicity for MNPA at the tested dose. Upon termination of the bioassay, the MNPA-treated animals were found to have tumors of the lung, liver, nasal cavity, forestomach and kidneys. The control animals showed no tumors in these organs. The incidence of lung tumors in the MNPA group was statistically significant (P < 0.025). The results of this study show that MNPA is a carcinogen in K344 rats.