1. T.l.c. of the 24-48 h urine of rats dosed with 4-aminobiphenyl (ABP) showed that 4-acetylaminobiphenyl (AABP), 4'-hydroxy-4-aminobiphenyl(4'-hydroxy-ABP), 2'-hydroxy-4-acetylaminobiphenyl(2'-hydroxy-AABP), 4'-hydroxy-4-acetylaminobiphenyl(4'-hydroxy-AABP), 3'-hydroxy, 4'-methoxy-4-acetylaminobiphenyl (3'-hydroxy-4'methoxy-AABP), 4'-hydroxy, 3'-methoxy-4-acetylaminobiphenyl (4'-hydroxy-3'-methoxy-AABP), and 3',4'-dihydroxy-4-acetylaminobiphenyl(3',4'-dihydroxy-AABP) are urinary metabolites. Neither 4-nitrosobiphenyl(nitroso-BP) nor N-hydroxy-4-acetylaminobiphenyl(N-hydroxy-AABP) were detected. 2. Radiochromatography of the 48-h urine of rats dosed with 14C-ABP gave three fractions, U1, U2 and U3 containing 34.6%, 38.8% and 20.4%, respectively, of the total 14C. The conjugated metabolites of ABP were found in U1 and U2, and the unconjugated metabolites in U3, indicating that 80% of the 14C activity in urine was in conjugated, and only 20% in unconjugated metabolites. 3. Rechromatography of U3 gave six radioactive bands from which the following metabolites were isolated and identified as being 4'-hydroxy-AABP, 3'-hydroxy-4'-methoxy-AABP, 4'-hydroxy-3'-methoxy-AABP, 3',4'-dihydroxy-AABP, AABP, ABP, 4,4'-bisazoxybiphenyl (BABP) and 4-(4-aminophenyl)-1,2-benzoquinone. Neither nitroso-BP nor N-hydroxy-AABP were detected.