Glutamine synthetase, glutamate synthase and glutamate dehydrogenase activities were measured in the cytosol extracted from developing effective soybean root nodules and compared to the activities found in cytosol extracted from ineffective soybean root nodules and soybean root tissue. In the cytosol of effective soybean root nodules glutamine synthetase activity was found to increase up to floral initiation while glutamate dehydrogenase activity decreased. Glutamate synthase activity was detected but it did not increase as much as glutamine synthetase. In ineffective nodule cytosol and root tissue much lower levels of glutamine synthetase and similar levels of glutamate dehydrogenase, as compared to effective nodule cytosol, were found but no glutamate synthase was detectable. The results indicated that glutamine synthetase/glutamate synthase cycle is operative for ammonia assimilation in effective soybean root nodules and glutamate dehydrogenase probably has no synthetic role.