Studies were performed to assess covalent binding of [3H]benzidine, [14C]N-acetylbenzidine, [14C]N, N'-diacetylbenzidine, and the benzidine-derived azo dye Direct Blue 6 to rat hepatic DNA. Following IP injection into male Sprague-Dawley rats, benzidine and N-acetylbenzidine bound to liver DNA to yield the same adduct: N-(deoxyguanosin-8-yl)-N'-acetylbenzidine. The isomeric N-(deoxyguanosin-8-yl)-N-acetylbenzidine and the deacetylated adduct N-(deoxyguanosin-8-yl) benzidine were also synthesized, but neither of these adducts was detected in vivo. Injection of N,N'-diacetylbenzidine resulted in only barely detectable binding which was insufficient for adduct analysis. [3H]Direct Blue 6 was administered to male Wistar rats either by IP injection or by gavage. In both instances, Direct Blue 6 bound covalently to liver DNA; however, binding occurred at a much higher level in the IP injected animals. With IP injected animals, high pressure liquid chromatographic analysis indicated that approximately 70% of the radioactivity was associated with N-(deoxyguanosin-8-yl) benzidine.