Soybean [Glycine max (L.) Merr.] yields are influenced by the nutrient content of the soil which might, in turn, be reflected by the ability of roots to efflux hydrogen ions (H+). Regulation of H+ efflux by NADH oxidation was investigated to evaluate the physiological response of H+ efflux and its relation to soybean yield among different cultivars. Enzyme assays and pH measurements were used to monitor NADH oxidation and H+ efflux by soybeans grown in a controlled-environmental chamber. Nutrient content, and yields of biomass and seed were measured with field-grown soybeans. Dry matter yields were determined for relative treatment comparisons in all experiments. Dry weights of different soybean cultivars were negatively correlated to NADH oxidation and positively correlated to H+ efflux. Comparison of 'Davis' and 'Forrest' soybeans indicated that dry weight, seed yield, and nutrient-ion contents were negatively correlated to NADH oxidation and positively correlated to H+ efflux. These fidings suggest that soybean nutrition and yield are closely interrelated with oxidation-reduction reactions in the plant's root tissues. Favorable growth and yield performance appears to require a supply of H+ ions. The efflux of these ions, in turn, appears to favor nutrient uptake and hence growth and yield potential. The chemical supplier of H+ ions remains obscure and NADH might or might not be a contributor.