Cytochrome b₅₆₁ from bovine adrenal chromaffin vesicles contains two hemes b with EPR signals at gsubscript z = 3.69 and 3.14 and participates in transmembrane electron transport from extravesicular ascorbate to an intravesicular monooxygenase, dopamine {szligbeta}-hydroxylase. Treatment of purified cytochrome b₅₆₁ in an oxidized state with a sulfhydryl reagent, 4,4'-dithiodipyridine, caused the introduction of only one 4-thiopyridine group per b₅₆₁ molecule at either Cys57 or Cys125. About half of the heme centers of the modified cytochrome were reduced rapidly with ascorbate as found for the untreated sample, but the final reduction level decreased to approximately65%. EPR spectra of the modified cytochrome showed that a part of the gsubscript z = 3.14 low-spin EPR species was converted to a new low-spin species with gsubscript z = 2.94, although a considerable part of the heme center was concomitantly converted to a high-spin g = 6 species. Addition of ascorbate to the modified cytochrome caused the disappearance or significant reduction of the EPR signals at gsubscript z = 3.69 and 3.14 of low-spin species and at g = 6.0 of the high-spin species, but not for the gsubscript z approximately 2.94 species. These results suggested that the bound 4-thiopyridone at either Cys57 or Cys125 affected the intravesicular heme center and converted it partially to a non-ascorbate-reducible form. The present observations suggested the importance of the two well-conserved Cys residues near the intravesicular heme center and implied their physiological roles during the electron donation to the monodehydroascorbate radical.