Hepatic cytosols catalyzed a 3'-phosphoadenosine 5'-phosphosulfate (PAPS)-dependent O-sulfonation of N-hydroxy-N-methyl-4-aminoazobenzene (N-HO-MAB) and of several other N-hydroxy arylamines. The presumed product from N-HO-MAB, N-methyl-4-aminoazobenzene-N-sulfate, reacted with added guanosine to yield N-(guanosin-8-yl)-N-methyl-4-aminoazobenzene, with methionine to form a sulfonium derivative that decomposed to yield 3-methylmercapto-N-methyl-4-aminoazobenzene, and with ribosomal RNA to give a bound derivative. N-Methyl-4-aminoazobenzene was converted to N-(guanosin-8-yl)-N-methyl-4-aminoazobenzene in concerted N-oxidation and O-sulfonation reactions conducted aerobically with a fortified 10,000 X g rat liver supernatant. In the absence of an added nucleophile, metabolically formed N-methyl-4-aminoazobenzene-N-sulfate (or the nitrenium ion from this unstable ester) was reduced by N-HO-MAB to form N-methyl-4-aminoazobenzene; the N-HO-MAB was oxidized, probably through a nitrone intermediate, to yield products that included N-hydroxy-4-aminoazobenzene and formaldehyde. An analogous reaction was noted between N-benzoyloxy-N-methyl-4-aminoazobenzene and N-HO-MAB in the absence of cytosol and PAPS. Hepatic N-HO-MAB sulfotransferase activities were in the order: male rat greater than female rat, male rabbit, male guinea pig, male mouse greater than male hamster. Male rat kidney and small intestine cytosols had low activities; the other tissues studied had little or no activity. Hepatic sulfotransferase activities for N-HO-MAB and N-hydroxy-N-acetyl-2-aminofluorene displayed different pH optima and inhibitor and activator responses. The rates of PAPS-dependent rat liver cytosol-catalyzed esterification of N-hydroxy-N-ethyl-4-aminoazobenzene, N-hydroxy-4-aminoazobenzene, and N-hydroxy-1- and 2-naphthylamine were 20 to 50% of that for N-HO-MAB. Activities for trans-N-hydroxy-4-aminostilbene, N-hydroxy-2-aminofluorene, N-hydroxyaniline, and N-hydroxy-N-methyl-N-benzylamine were not detected. No microsomal reduced nicotinamide adenine dinucleotide-dependent reduction or reduced nicotinamide adenine dinucleotide phosphate-dependent oxidation or cytosolic transferase reactions for N-HO-MAB, except the above-described PAPS-dependent reaction, were detected in rat liver.