Presence of certain nitrogenous compounds in the growth medium significantly enhanced the salt tolerance of the fresh-water cyanobacterium Anabaena sp. strain L-31 as well as the brackish water cyanobacterium Anabaena torulosa. Among these, nitrate, ammonium, and glutamine were most effective followed by glutamate and aspartate. These nitrogenous compounds also inhibited Na+ influx in both Anabaena spp. with the same order of effectiveness as that observed for protection against salt stress. The inhibition of Na+ influx on addition of the nitrogenous substances was rapid; nitrate and ammonium inhibited Na+ influx competitively. Proline and glycine did not affect Na+ influx and also had no influence on the salt tolerance of either Anabaena sp. The observed protection was not consequent to a stimulatory effect of combined nitrogen on growth per se. Uptake of NO3− and NH4+ increased during salt stress but was not correlated with growth. Intracellular levels of NO3− and NH4+ were found to be inadequate to constitute a major component of the internal osmoticum. The results suggest that inhibition of Na+ influx by combined nitrogen is a major mechanism for protection of cyanobacteria against salt stress.