Oxygen uptake of the microorganisms cultured from river water were examined to find the effect of heavy metals with or without a substrate. Without the addition of a substrate, effect of a metal was examined in median toxic concentrations of oxygen uptake (TC₅₀), and the metal toxicity decreased in the order of Hg²⁺, Cu²⁺, Ni²⁺, Cd²⁺, Zn²⁺, Al³⁺, Pb²⁺, Fe³⁺, Mn²⁺. When a substrate was added, the effect of metals appeared at a higher concentration. The modified Wiegand equation was applied to analyze the form of oxygen uptake inhibition caused by cadmium. The decrease of L and y₀ values was indicated without substrate added, and when the substrate was added, to the effect of cadmium was different from the former. Dehydrogenase activity was related the decrease of oxygen uptake in the absence of a substrate.