Abstract:The NaCl-induced inhibition of nitrogen-fixing activity of blue-green algae Anabaena 7120 was removed to a certain degree by the. introduction of appropriaternannitol into the test system. When the preincubation of algae cells was done in dark before experiment for 24 hours or in the presence of photosynthetic inhibitors, such as DNP or CCCP, the enhancing effect of mannitol on the nitrogen-fixing activity of Anabaena 7120 supressed by NaCl weakened or disappeared. In the addition of O2 together with H2 the enhancing effect of mannitol on nitrogen-fixing activity of Anabaena 7120 was reinforced under NaCl stress. A marked inaction in nitrogen-fixing activity of Anabaena 7120 occurred under NaCl stress when O2 was introduced into the test system alone. When algae cells inactivited by NaCl were incubated in the aerobic condition the accelerated action of mannitol upon the nitrogen-fixing activity was higher than that in anaerobic condition. The mannitol-supported nitrogen-fixing activity of Anabaena 7120 under NaCl stress was accelerated in the addition of N2 together with CO2 or addition of CO2 alone, while it was depressed by adding N2 alone.