The metabolism of the carcinogen benzo(a)pyrene was determined, using rhesus monkey hepatic and pulmonary microsomal enzymes. Metabolites were separated by high pressure liquid chromatography and identified using known reference standards. Metabolites were quantitated by scintillation spectrometry. Liver and lung microsomes metabolized benzo(a)pyrene to the following metabolites: 9,10-, 7,8- and 4,5-dihydrodihydroxybenzo(a)pyrene; benzo(a)pyrene-1,6-dione, -3,6-dione and -6,12-dione; and 9- and 3-hydroxybenzo(a)pyrene. Two unidentified metabolites and 1 metabolite region which chromatographed prior to 9,10-dihydrodihydroxybenzo(a)pyrene were produced by liver and lung microsomes. The 2 unknown peaks were located between 9,10- and 4,5-dihydrodihydroxybenzo(a)pyrene. Two additional unknown metabolites were produced only in the liver and had retention times greater than the 4,5- and 7,8-dihydrodihydroxybenzo(a)pyrene metabolites, respectively. Quantitative determination of benzo(a)pyrene metabolism revealed large differences for the 3 monkeys and the respective tissue activities. Liver activity for each animal was substantially higher than lung activity for all benzo(a)pyrene metabolites. The ratio of the metabolites also differed between the liver and lung. 3-Hydroxy-benzo(a)pyrene represented over 60% of the total liver metabolite fraction and 30% of the total lung metabolite fraction. The total quinone fraction represented between 7 and 13% of the total metabolites in the liver and comprised over 40% of the total lung metabolites. The metabolite ratios for the dihydrodiols were similar for both tissues.