The catalytic reduction of nickel ions on the hanging mercury drop electrode in the presence of traces of 6-mercaptopurine-9-D-riboside (MPR) was investigated by linear scan voltammetry in the physiological pH range. A nickel complex is reduced at potentials mon positive than the reduction of the hydrated ion. The reduction of the complex produces a voltammetric pattern which is typical for the electrode processes controlled by a parallel chemical reaction. The theoretical treatment for the regeneration of the reactant by a redox reaction was adapted to the investigated electrode process. The kinetic equation thus derived was used for the determination of both the rate (k(1) = 3.11 x 10(6) mol(-1) dm(3) s(-1)) and formation (log beta(1) = 3.40) constants for the reducible complex by means of a multiparametric curve fitting approach. Under the same conditions traces of MPR catalyze the oxidation of Ni amalgam producing both an increase of the anodic peak current and the shift of this peak towards more negative potentials.