We have compared the characteristics of nitrate uptake by Aphanothece halophytica grown under non-stress and salt-stress conditions. Both cell types showed essentially similar patterns of nitrate uptake toward ammonium, nitrite, and DL-glyceraldehyde. Although the affinities of nitrate to non-stress cells and salt-stress cells were not significantly different, i.e., K-s = 416 and 450 muM respectively, the V-max value for non-stress cells was about twofold of that for salt-stress cells (9.1 vs 5.3 mumol min(-1) mg(-1) Chl). Nitrate uptake by A. halophytica was found to be dependent on Na+. Ammonium inhibited nitrate uptake, and the presence of methionine sulfoximine could not release the inhibition by ammonium. Nitrite appeared to competitively inhibit nitrate uptake with a K-i value of 84 muM. Both chloride and phosphate anions did not affect nitrate uptake. DL-Glyceraldehyde, an inhibitor of CO2 fixation, caused a reduction in the uptake of nitrate.