The activities of the following enzymes were studied in connection with dinitrogen fixation in pea bacteroids: glutamine synthetase (L-glutamate:ammonia ligase (ADP-forming)) (EC 6.3.1.2) (GS); glutamate dehydrogenase (NADP+) (L-glutamate:NADP+ oxidoreductase (deaminating)) (EC 1.4.1.4) (GDH); glutamate synthase (L-glutamine:2-oxoglutarate aminotransferase (NADPH-oxidizing)) (EC 2.6.1.53) (GOGAT). GS activity was high throughout the growth of the plant and GOGAT activity was always low. It is unlikely that GDH or the GS–GOGAT pathway can account for the incorporation of ammonia from dinitrogen fixation in the pea bacteroid.