Both nitrite reductase and nitrate reductase were induced by nitrite, but there were differences in the time course of induction and in the response to different NO2 - concentrations between these enzymes. NH4 + depressed the induction of nitrite reductase. NADH2 dependent glutamate dehydrogenase activity was enhanced by those NO2-concentrations in the medium at which unmetabolized NO2 - occurred in the roots. NADPH2 and NAD+ dependent GDh activities were not affected. In vivo modification and (or) in vivo activation were probably responsible for the increase in NADH2 dependent GDH activity.