The presence of tetrahydro-beta-carbolines and beta-carbolines was studied in raw, cooked and smoked fish and meat. 1,2,3,4-Tetrahydro-beta-carboline-3-carboxylic acid (THCA) usually was the major beta-carboline found, whereas 1-methyl-1,2,3,4-tetrahydro-beta-carboline-3-carboxylic acid (MTCA) appeared in smoked and 'well done' cooked samples. THCA was detected in raw fish (nd-2.52 micrograms/g), cooked fish (nd-6.43 micrograms/g), cooked meats (nd-0.036 microgram/g), smoked fish (0.19-0.67 microgram/g) and smoked meats (0.02-1.1 micrograms/g). Smoked and cooked samples contained higher amounts of THCA and MTCA than raw products. Deep cooking of fish and meat increased both THCA and MTCA, and this was accompanied by the formation of more beta-carbolines, norharman and harman. The tetrahydro-beta-carbolines THCA and MTCA were chemical precursors of the co-mutagens norharman and harman during cooking. These and previous results confirm that foods are an important source of beta-carbolines in humans.