Dopamine beta-monooxygenase is shown to catalyze the oxidation of N,N,N',N'-tetramethyl-1,4-phenylenediamine (TMPD) to its cation radical in the presence of a regular substrate and molecular oxygen. The enzyme-mediated oxidation of TMPD is stoichiometrically coupled with the hydoxylation of the substrate to the corresponding enzymatic product. TMPD is kinetically well behaved as an alternate electron donor for the enzyme with a potency comparable to that of the most efficient electron donor, ascorbate. Dopamine beta-monooxygenase mediated oxidation of TMPD has been employed to design a convenient and sensitive spectrophotometric assay for the enzyme. The finding that TMPD is a well behaved facile alternate electron donor for dopamine beta-monooxygenase raises some interesting novel questions regarding the specificity and chemistry of the reduction site, which may have important implications on the reduction of active site coppers of the enzyme.