Chronoamperometry based on the 'diffusion' layer concept of the convective system was used to assay the activity of mitochondrial malate dehydrogenase (MDH). When the enzyme-catalysed reaction was initiated by adding the enzyme MDH into a well-stirred nicotinamide adenine dinucleotide (NADH, coenzyme) solution, the enzyme activity could be indicated by the continuous in-situ decrease in the limiting steady-state oxidation current of NADH. The effects of some lanthanide ions on the MDH activity were monitored. The La3+, Ce3+ and Eu3+ ions could activate markedly the enzyme MDH as the concentrations were lower. The activation mechanism would be that the lanthanide ions could interrupt the binding of NAD(+) to MDH by combining preferentially to NAD(+). This mechanism was proposed on the basis of voltammetric and Raman spectroscopic studies.