Summary.The influence of the nitrogen supply on the growth and development of quackgrass (Agropyron repensL. Beauv.) was investigated by growing plants in sand culture at nitrogen levels of 210, 10–5, 5.25 and 2.6 ppm in addition to constant levels of other basic nutrients.When these nutrient regimes were first applied the rate of leaf emergence showed a positive correlation with the nitrogen level. This effect, however, did not persist and the rate subsequently attained was similar at all levels of nutrition. Tiller emergence, on the other hand, was markedly dependent on the nitrogen supply and at 2.6 ppm N was almost completely suppressed. As a result of this effect the production of secondary rhizomes, i.e. those arising from tillers, was correspondingly reduced. There was, however, a slight increase in the number of rhizomes produced from buds on the parent shoot.At 2.6 ppm N the dry weight of the shoots was reduced by approximately 75% as compared with the highest nitrogen level but there was no significant reduction in the weight of the roots or rhizomes. The shoot/root ratio was thus considerably decreased at the lower ni trogen levels.By varying the nitrogen supply it was also possible to control quite precisely the behaviour of the rhizome buds. At 2.6 ppm and 10.5 ppm the buds ceased to grow when 2–6 mm in length, the size attained being determined by the nitrogen level. At 210 ppm, however, the onset of dormancy was eliminated and all of the buds grew out as lateral branches. Bud growth could also be induced at a low nitrogen level (5.25 ppm) by removing the rhizome apex.The significance of these results is discussed in relation to the ecology of the plant and to the problem of apical dominance.Influence de diverses dases d'azote sur la croissarce et le développent d'Agropyron repensL. B