Application of allopurinol (AP; 1H-pyrazolo-[3,5-D]pyrimidine-4-ol) to intact nodulated roots of ureide-forming legumes causes rapid inhibition of NAD:xanthine dehydrogenase (XDH; EC 188.8.131.52), cessation of ureide synthesis and, subsequently, severe nitrogen, deficiency (Atkins et al. 1988. Plant Physiology 88: 1229-1234). Nitrogen deficiency is a result of inhibited nitrogenase (EC 184.108.40.206) activity. Using an open gas exchange system to measure H-2 and CO2 evolution, short term effects of AP application were examined in a Hup- soybean symbiosis [Glycine max (L.) Merr. cv. Harosoy: USDA 16]. The onset of inhibition of nitrogenase was detected after ca 2 h exposure of the roots to AP. At the same time xanthine began to accumulate and ureide levels declined in nodules as a result of inhibition of XDH. The decline in H-2 evolution following AP application was not due to altered electron allocation between N2 and H+ by nitrogenease but was coincident with increased gaseous diffusive resistance of nodules and a decline in intracellular oxygen concentration. A possible scheme for the intermediary metabolism of soybean nodules which might account for a direct connection between nitrogenase activity and ureide synthesis is proposed. The suggested mechanism envisages coupling production of reducing power by cytosolic enzymes of purine oxidation to synthesis of dicarboxylic acid substrates (malate and succinate) required for bacteroid respiration.