1. Rats dosed with nitrosobenzene (56 mumol/kg), 4-chloronitrosobenzene (53 mumol/kg), 3,4-dichloronitrosobenzene (53 mumol/kg), 4-ethoxynitrosobenzene (86 mumol/kg), 4-nitrosobiphenyl(nitroso-BP, 55 mumol/kg) or 2-nitrosofluorene (256 mumol/kg) had maximal ferrihaemoglobin (HbFe3+) concn of 69, 68, 69, 67, 55 and 42% after 15, 25, 48, 35, 80 and 115 min, respectively, indicating differences in solubility of the nitrosoarenes in body fluids. 2. Nitroso-BP and 3-hydroxy-4-aminobiphenyl (3-hydroxy-ABP) catalytically oxidized HbFe2+ in bovine erythrocytes in vitro; nitroso-BP was three times as active as 3-hydroxy-ABP. 3',4'-Dihydroxy-4-aminobiphenyl (3',4'dihydroxy-ABP) showed only low catalytic activity, and seven other ABP metabolites exhibited only marginal activity. 3. Nitroso-BP was inactive in solutions of purified human Hb, but 3-hydroxy-ABP catalytically oxidized HbFe2+, indicating that nitrosoarenes oxidize HbFe2+ in erythrocytes in vitro and in vivo by a mechanism different from that of o-aminophenols. The second-order rate constant for HbFe2+ oxidation by 3-hydroxy-ABP at 37 degrees C was k2 = 19.1 +/- 1.31/mol per s.