The effect of NaCl and Na(2)SO(4) salinity on NO(3) (-) assimilation in young barley (Hordeum vulgare L. var Numar) seedlings was studied. The induction of the NO(3) (-) transporter was affected very little; the major effect of the salts was on its activity. Both Cl(-) and SO(4) (2-) salts severely inhibited uptake of NO(3) (-). When compared on the basis of osmolality of the uptake solutions, Cl(-) salts were more inhibitory (15-30%) than SO(4) (2-) salts. At equal concentrations, SO(4) (2-) salts inhibited NO(3) (-) uptake 30 to 40% more than did Cl(-) salts. The absolute concentrations of each ion seemed more important as inhibitors of NO(3) (-) uptake than did the osmolality of the uptake solutions. Both K(+) and Na(+) salts inhibited NO(3) (-) uptake similarly; hence, the process seemed more sensitive to anionic salinity than to cationic salinity.Unlike NO(3) (-) uptake, NO(3) (-) reduction was not affected by salinity in short-term studies (12 hours). The rate of reduction of endogenous NO(3) (-) in leaves of seedlings grown on NaCl for 8 days decreased only 25%. Nitrate reductase activity in the salt-treated leaves also decreased 20% but its activity, determined either in vitro or by the ;anaerobic' in vivo assay, was always greater than the actual in situ rate of NO(3) (-) reduction. When salts were added to the assay medium, the in vitro enzymic activity was severely inhibited; whereas the anaerobic in vivo nitrate reductase activity was affected only slightly. These results indicate that in situ nitrate reductase activity is protected from salt injury. The susceptibility to injury of the NO(3) (-) transporter, rather than that of the NO(3) (-) reduction system, may be a critical factor to plant survival during salt stress.