The demethylation of carcinogenic N-nitrosodimethylamine (NDMA) and N-nitrosomethylaniline (NMA) is catalyzed by horseradish peroxidase in the presence of hydrogen peroxide. NMA is a better substrate for peroxidase than NDMA. The Km values are 0.74 and 3.12 mmol/l for NMA and NDMA, respectively. The oxidation of NDMA and NMA is inhibited by radical trapping agents (nitrosobenzene, glutathione, ascorbate, NADH). This indicates the radical mechanism for the peroxidase-mediated oxidation of both N-nitrosamines. The in vitro metabolism of NMA using peroxidase was investigated in detail. Beside formaldehyde, the metabolites formed from NMA by peroxidase include aniline, p-aminophenol and phenol. Phenol formation presumably arose from N-demethylation of NMA via a benzenediazonium ion (BDI) intermediate while aniline and p-aminophenol from denitrosation of this carcinogen. The results are discussed from the point of view of the role of peroxidases in the metabolism of N-nitrosamines.