1-Methyl-1,2,3,4-tetrahydro-beta-carboline-3-carboxylic acid (MTCA) and 1,2,3,4-tetrahydro-beta-carboline-3-carboxylic acid (TCCA), both precursors of mutagenic N-nitroso compounds (N-nitrosamines, 1-methyl-2-nitroso-1,2,3,4-tetrahydro-beta-carboline-3-carboxylic acid and 2-nitroso-1,2,3,4-tetrahydro-beta-carboline-3-carboxylic acid), were detected in various food-stuffs, urine from healthy human subjects and human milk. A purification procedure, involving a chemically-bonded material followed by HPLC combined with fluorometric detection, was used for the quantitative determination of these compounds, allowing the separation of two diastereoisomers of MTCA. An HPLC and mass spectrometry method was also developed for their identification. Comparing the concentration of MTCA and TCCA in fermented products and raw materials suggested that tetrahydro-beta-carbolines may have been produced through fermentation or by condensation of tryptophan and acetaldehyde formed from ethanol added as a food preservative. This is the first report of excretion of tetrahydro-beta-carbolines in human urine and human milk. A comparison of the concentrations of tetrahydro-beta-carbolines in urine from human infants and human milk indicates that tetrahydro-beta-carbolines may be synthesized endogenously in humans. A possible pathway of tryptophan metabolism in plants and animals is presented.