Six azodyes derived from benzidine, o-tolidine or o-dianisidine were separately administered orally by gavage to rats. Urine was collected over a 24 h period. Following dichloromethane extraction, urines were analysed by h.p.l.c. for the presence of the respective parent amine and its N-acetylated and N, N' -diacetylated derivatives. After alkaline hydrolysis, urines were analysed for the amines resulting from the cleavage of N-conjugates. All six dyes, direct black 38, direct brown 95, direct blue 6, Congo red, trypan blue and Chicago sky blue were found to be reduced, N-acetylated and N-conjugated. However, no N, N' -diacetylated metabolites were detected. After administration of the same dyes via injection into the hepatic portal vein, bile was collected over a 3 h period by cannulation of the bile duct. Urine was withdrawn from the bladder by syringe at the end of the three hours. Both body fluids were analysed for reduction products which were found only in the case of direct black 38, direct brown 95 and direct blue 6. Of the six dyes examined only the three direct dyes were mutagenic to S. typhimurium strains TA98 and TA1538 in the absence of flavin mononucleotide. The same three dyes were also substrates for rat liver microsomal azoreductase enzymes whereas Congo red, trypan blue and Chicago sky blue were shown to be inactive in a previous publication. The possible relationship between these results and the potent carcinogenicity exhibited by direct black 38, direct blue 6 and direct brown 95 is discussed.