A high rate of nitrate uptake was observed in Nostoc muscorum when cells were grown on elemental nitrogen as compared to that when they were grown on nitrate or ammonium. The uptake of nitrate was light dependent. However, supplementation with ATP (50 microM) stimulated nitrate uptake both in light and darkness. ADP, under similar conditions had no effect. 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), 2-n-heptyl-4-hydroxyquinoline, (HOQNO) and KCN inhibited nitrate uptake in light which could be partially reversed by addition of ATP. Inhibition by carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP), an uncoupler of photophosphorylation, was complete and could not be restored by the addition of ATP. N,N'-dicyclohexylcarbodiimide (DCCD), a specific inhibitor of ATPase, blocked nitrate uptake in the presence or absence of externally added ATP. Although no nitrate uptake was observed under anaerobic conditions in dark, addition of ATP resulted in uptake of nitrate, which was similar in magnitude to that observed under aerobic condition in the light, and was inhibited by DCCD. Ammonium ions inhibited the uptake of nitrate in the absence of ATP but in its presence there was simultaneous uptake of nitrate and ammonium ions. However, uptake of ammonium ions alone was not affected by presence or absence of ATP in the external medium. It was concluded that nitrate ion uptake was energy dependent and may be linked with a proton gradient which can be formed either by photophosphorylation or ATP hydrolysis.