Betel quid is chewed as a masticatory material by people in certain areas of Asia. The quid chewing has been related to oral cancer by epidemiological study. The mutagenic components in the aqueous extracts of betel quid ingredients were studied. Only nitrite-treated aqueous extract of Piper betle L fruits, leaves or rhizoma were demonstrated to exhibit a mutagenic response, using Salmonella typhimurium strains TA100 and TA1535 in the Ames test. When the aqueous extract of the fruit was nitrosated, the greatest number of mutagenic substances were formed at pH 3. The formation of mutagens was enhanced by increasing the temperature from 5 to 95 deg. C. Maximum production of the mutagens occurred within 15 min when nitrosation was conducted at 35 deg. C. The mutagenic components in nitrite-treated aqueous extract of Piper betle L fruit were found to be N-nitrosopiperidine, N-nitrosopyrrolidine, N-nitrosomorpholine, and other compounds, as determined by gas chromatography-thermal energy analyzer.