Gene expression in pea roots grown in a medium with a low oxygen concentration was compared with that in nitrogen-fixing pea root nodules induced by Rhizobium bacteria. The results show that during microaerobiosis the expression of eight genes is increased. None of these belong to the group of genes earlier identified as nodulin genes. On the other hand, no enhanced transcription of microaerobic genes can be detected during nodule development and hybridizations of Northern blots, containing nodule RNA and RNA isolated from oxygen-stressed roots, show that the alcohol dehydrogenase genes are not expressed at a higher level in pea root nodules whereas a higher expression is observed during microaerobiosis. From these observations it can be concluded that it is unlikely that a low concentration of free oxygen induces the expression of nodulin genes. Furthermore, genes that are activated as a result of oxygen deficiency are not expressed in pea root nodules, indicating that if the concentration of free oxygen is low the nodule cells do not suffer under microaerobic conditions. Probably, leghemoglobin functions as an efficient oxygen buffer for the energy-generating process in both the plant cells and the bacteroids.