Nitrogen-starved sunflower plants (Helianthus annuus L. cv. Peredovic) cannot absorb NO 3 (-) or NO 2 (-) upon initial exposure to these anions. Ability of the plants to take up NO 3 (-) and NO 2 (-) at high rates from the beginning was induced by a pretreatment with NO 3 (-) . Nitrite also acted as inducer of the NO 2 (-) -uptake system. The presence of cycloheximide during NO 3 (-) -pretreatment prevented the subsequent uptake of NO 3 (-) and NO 2 (-) , indicating that both uptake systems are synthesized de novo when plants are exposed to NO 3 (-) . Cycloheximide also suppressed nitrate-reductase (EC 1.6.6.1) and nitrite-reductase (EC 1.7.7.1) activities in the roots. The sulfhydryl-group reagent N-ethylmaleimide greatly inhibited the uptake of NO 3 (-) and NO 2 (-) . Likewise, N-ethylmaleimide promoted in vivo the inactivation of nitrate reductase without affecting nitrite-reductase activity. Rates of NO 3 (-) and NO 2 (-) uptake as a function of external anion concentration exhibited saturation kinetics. The calculated Km values for NO 3 (-) and NO 2 (-) uptake were 45 and 23 μM, respectively. Rates of NO 3 (-) uptake were four to six times higher than NO 3 (-) -reduction rates in roots. In contrast, NO 2 (-) -uptake rates, found to be very similar to NO 3 (-) -uptake rates, were much lower (about 30 times) than NO 2 (-) -reduction rates. Removal of oxygen from the external solution drastically suppressed NO 3 (-) and NO 2 (-) uptake without affecting their reduction. Uptake and reduction were also differentially affected by pH. The results demonstrate that uptake of NO 3 (-) and NO 2 (-) into sunflower plants is mediated by energy-dependent inducible-transport systems distinguishable from the respective enzymatic reducing systems.