Hydrogen uptake has been shown to occur with pea root nodule breis and this uptake has been shown to be confined to the bacteriods. The uptake of hydrogen by washed bacteriods, in the absence of any added substrates, has been shown to be accompanied by oxygen uptake and the ratio of hydrogen uptake to oxygen uptake in these preparations has been found to be 2:1. Substrates, provided to washed bacteriods, inhibit the uptake of hydrogen and it has been found that the utilisation of substrates, as measured by carbon dioxide evolution, is inhibited by hydrogen. It is suggested that hydrogen and substrates compete for electron carriers and that electrons from the hydrogen reduce components of the electron transport pathway and ATP is produced.