Nitrogen regulation of C-14-arginine uptake and arginase (EC 3.5.3.1) have been studied in the cyanobacterium Anabaena cycadeae and its mutant strain lacking glutamine synthetase (GS) activity. Development of C-14-arginine uptake and arginase systems started simultaneously, and both systems were found to be independent. The C-14-arginine uptake and arginase activity systems in both strains were found to be NH4+ repressible, and NH4+ was the initial repressor signal. However, glutamine could not inhibit the C-14-arginine uptake and arginase activity, suggesting that repression of C-14-arginine uptake and arginase by NH4+ does not require the operation of GS and that NH4+ itself is the repressor signal.