In an effort to elucidate the mechanism of the initiation reaction of the denaturation of oxyhemoglobin, I, to methemoglobin, II, by hydrazines, we have investigated by electron paramagnetic resonance, EPR, and visible spectrophotometry at 22 degrees C, pH 7.4, the reaction of I with 1,1-diphenyl-2-picrylhydrazine, III, and 2,2-diphenyl-1-picrylhydrazyl, IV, in dimethylsulphoxide/buffer and methanol/buffer mixtures, these organic solvents included at a concentration of 10 v/v% to render III and IV soluble while not causing appreciable denaturation of I. In both buffer mixtures, the results obtained were the same. For the I/III reaction mixtures, although the spectrophotometric data obtained showed denaturation to occur, there was no EPR evidence for formation of IV, contrary to expectation based on the chemical structure of III. The EPR observations on each I/IV reaction mixture showed a rapid decrease in IV signal intensity to a value that, depending on the initial reactant concentrations, was either below the detection limit or, when measurable, constant with time. The results of similar EPR measurements on analogous II/IV reaction mixtures were the same. These EPR results are compatible with the idea that IV forms a complex with the protein moiety of I and II, and show that the I/III reaction could yield IV and thus involve a one-electron transfer process.