[14C]Benzo[a]pyrene ([14C]B[a]P) injected intraperitoneally into rats appeared rapidly in liver, lung and kidney, and remained detectable in these tissues for at least 7 days. A large proportion (7--13%) of the 14 C became covalently bound to tissue macromolecules, probably primarily proteins. Subcellular organelles of the liver were all found to bind the carcinogen, the microsomes most rapidly and the light mitochondrial fraction taking up 14C later. Nuclear bound 14C was detected in both liver and lung. Purification of the cytosolic 14C from liver revealed specific binding to the same cytosolic proteins purified from the in vitro reaction of [14C] B[a]P.