Many mutagenic heterocyclic amines (HAs) have been isolated from cooked foods and pyrolysates of amino acids and proteins, and the carcinogenicity of 10 of these HAs in rodents and of 1 in monkeys has been reported. Quantification of these carcinogenic HAs in various kinds of cooked foods indicated that the level of 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) was highest (0.56-69.2 ng/g), that of 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx) was second highest (0.64-6.44 ng/g), and those of other HAs were 0.03-2.50 ng/g. Heterocyclic amines were found in urine samples of 10 healthy volunteers consuming a normal diet, but HAs were not detectable in urine samples of three patients receiving parenteral alimentation. These results strongly suggest that humans are continuously exposed to HAs derived from food in the normal diet. Based on quantitative data on the levels of HAs in cooked foods and urine samples, the daily exposures to PhIP and MeIQx were estimated to be 0.1-13.8 micrograms and 0.2-2.6 micrograms per person, respectively. These levels of carcinogenic HAs are in the same range as those of other carcinogens such as N-nitrosodimethylamine and benzo[a]pyrene to which humans are exposed.