The hemoglobin (Hb) binding of five nitroarenes, i.e. nitrobenzene (NB), 4-nitrobiphenyl (4-NBP), 1-nitropyrene (1-NP), 2-nitronaphthalene (2-NN) and 2-nitrofluorene (2-NF), and the corresponding amines, administered p.o. to male S.D. rats, was determined by HPLC, to evaluate the extent of in vivo reductive and oxidative activations of these compounds to N-hydroxylamines, which covalently bind to Hb to form acid-labile sulfinamides. Hb binding of the nitroarenes, except for NB, was significantly lower than that of the corresponding amines. Among the aromatic amines, 4-aminobiphenyl exhibited extremely high Hb binding. Hb binding of NB and 4-NBP decreased markedly after pretreatment with a mixture of antibiotics, but the binding of the others did not decrease appreciably. 1-Aminopyrene and 1-NP bound abundantly to plasma proteins, although the Hb binding was slight. Based on the Hb binding and the in vitro metabolism by liver microsomes and intestinal bacteria, the extent of in vivo reductive activation of nitroarenes is discussed.