Protein content and activities of the enzymes glutamine synthetase (EC 6.3.1.2), NADH‐glutamate synthase (EC 1.4.1.14), NADH‐glutamate dehydrogenase (reductive amination (EC 1.4.1.2) and NAD+‐glutamate dehydrogenase (oxidative deamination) (EC 1.4.1.2) from the plant fraction of root nodules of alfalfa (Medicago sativa L. cv. Aragon) were determined under water stress. Only NADH‐glutamate synthase activity was inhibited during drought. The results indicate that the glutamine synthetase/NADH‐glutamate synthase cycle was fully operational in alfalfa nodules of control or even mildly stressed plants when N2‐fixation was not inhibited, but that the coupling between glutamine synthetase and NADH‐glutamate synthase was lost as drought progressed. Patterns of glutamine synthetase and NADH‐/NAD+‐gluta‐mate dehydrogenase activities reflect changes in ammonia content of nodules and/or availability of carbon substrates, and indicate that nodules maintain sufficient enzyme activity for ammonia assimilation throughout water stress.