Abstract: The addition of L-methionine-DL-sulpheximine (MSX) to the growing culture of Anabaena azotica resultes in releasing newly fixed NH3 into the medium and inhibits glutamine synthetase activity, showing a negative correlation between GS activity and ammonia production. It indicates that glutamine synthetase-glutamate synthetase pathway is essential to NH3 assimilation in A. azotica. When protein synthesis was inhibited by chloramphenicol, the addition of NH4+ decreased nitrogenase activity and glutamine synthetase activity, indicating inactivity or degradation of the NH4+-mediated enzyme. Under nitrogen-fixing condition, the halflife of the activity of nitrogenase was shorter then 4h, Whereas that of glutamine synthetase was longer than 10h. This suggests that glutamine synthetase may not be a positive regulator of nitrogenase. The inactivation effect of NH4+ or glutamine on nitrogenase increases with their concentration. However, there is no such relationship for GS. A low concentration of NH4+ (0.1—0.5 mmol/L. NH4Cl) and even a high concentration of gln (1.0—2.0 mol/L.) does not inhibit Gs activity. Thus, it suggests that nitrogenase is not directly regulated by GS. Furthemore, the depression of nitrogenase by NH4+ and gln can be eliminated by MSX, of which the function is in relation to the age of the algal culture.