We previously reported that inhibition of ethylene biosynthesis with aminoethoxyvinylglycine (AVG) eliminated the inhibitory effect of NO3– on nodulation of alfalfa (Medicago sativa L. cv. Aragon) plants grown aeroponically. In this work, the effect of Ag+, as an inhibitor of ethylene action, has been studied in plants growing aeroponically or in darkened tubes with vermiculite, and low-nitrate or high-nitrate solution. Vermiculite-grown plants developed up to 3 times as many nodules as did those growing aeroponically. Nodule formation was mirrored by dry-matter accumulation. High (10 mol m–3) NO3– applied from planting inhibited nodulation to an equal extent (c. 50%) in the two growth conditions. In contrast, Ag+ treatment increased nodule formation at all NO3– concentrations assayed under the two growth conditions, with the stimulation being higher in plants grown aeroponically. Finally, no effect of Ag+ (10 mmol m–3) on plant growth was observed in either of the growth conditions. The effectiveness of NO3– as a nodulation inhibitor and enhancer of ethylene biosynthesis in roots of alfalfa was also studied. Within 24 h after inoculation, 10 mol m–3 NO3– exerted most of its inhibitory effect on nodulation. At the same time, both 1-aminocyclopropane-1-carboxylic acid (ACC) oxidase activity and ethylene evolution rates markedly increased in inoculated and uninoculated alfalfa roots treated with NO3–. Support for a role of endogenous ethylene in the control of nodule formation in legumes is discussed.