Manganese is an essential trace element for human metabolism, but at higher concentrations it is a potent neurotoxin that presents clinical symptoms similar to those of Parkinson's disease. Since the toxicity of manganese may be related to its ability to accelerate the oxidation of catecholamines, we have examined the effect of aqueous Mn2+ on the formation and decay of the dopamine semiquinone radical ion. ESR spectroscopy was used to measure the kinetics of the disappearance of the semiquinone radical spectrum and the simultaneous appearance of the six-line spectrum of aqueous Mn2+ in Tris buffer. From the proposed mechanism for the autoxidation of dopamine to the quinone, the rate expression for semiquinone radical disappearance has the functional form -rate = k'[D(OH)2][Mn2+] at constant pH and molecular oxygen concentration, while the pH dependence is given by -log(rate) = log(constant) + (2 x pH), in agreement with the experimental results. The autoxidation of dopamine is catalyzed by manganese through the formation of a highly reactive complex. The effect of manganese is due to the fact that it can participate in a redox cycle which involves intramolecular electron transfer between manganese and the dopamine ligand.