Phytotron grown plants of Glycine max var. Caloria infected with Rhizobium japonicum 61-A-101 under controlled conditions as 14 d old seedlings develop a sharp maximum of nitrogenase activity of 13 ± 3 nmol C 2 H 4 · h -1 · mg nodule fresh weight -1 19 d after infection, followed by a long period of reduced activity (3 -5 nmol) between 30 and 45 days after infection. A higher maximum activity (18 nmol C 2 H 4 · h -1 · mg nodule -1 , lasting 7 days was found in Glycine max var. Mandarin, with a similar peroid of low activity (3 - 4 nmol) following, between 35 and 50 d after infection. Nitrogenase activity in the varieties infected with Rhizobium japonicum strain 3I1 b 85 is very similar. In both varieties, the leghaemoglobin continues to increase (3 fold) after the maximum nitrogenase activity is reached and starts to decline only after 35 to 40 d. Specific activities of the three enzymes aspartate aminotransferase (E.C.184.108.40.206.), glutamate dehydrogenase (E.C.220.127.116.11.) and alanine aminotransferase (E.C.18.104.22.168.) in the plant cell cytoplasm are changing very similarly to nitrogenase activity in the bacteroids, whereas the specific activities of the three enzymes in the bacteroids decrease only very slightly between 19 and 45 d. For these 3 enzymes the specific activity in the bacteroids during the phase of maximum nitrogenase activity is only 20 - 40% of the specific activity in the cytoplasm. A constant low activity (0.350 units) of glutamine synthetase (E.C.22.214.171.124.) is found in bacteroids from 19 to 45 d old nodules, whereas the specific activity in the plant cytoplasm increases from about 1.2 units at 19 d to more than 6 units at 45 d. The specific activity of GOGAT (E.C.1.4.13.) in bacteroids is 2 - 3 times higher than in the plant cell cytoplasm and increases slightly. Alanine dehydrogenase (E.C.126.96.36.199.) and 3-hydroxybutyrate dehydrogenase (E.C.188.8.131.52.) activities in bacteroids increase between 19 and 35 d after infection by a factor of 2 - 3. In 35 - 45 d old nodules, the specific activity of aianine-dh in bacteroids of the same Rhizobium japonicum strain (61-A-101) from plant var. Caloria is significantly smaller than in bacteroids from plant var. Mandarin, whereas for 3-hydroxybutyrate-dh the activity in bacteroids from var. Caloria is enhanced compared to bacteroids from var. Mandarin.