Nitrogen assimilation of pea plants (Pisum sativum cv. Meteor) was studied in growth cabinets tinder a range of water, salt, and aeration regimes in the rooting medium. Treatments were imposed in the period 14–30 d after germination using seedlings already nodulated in an optimum root environment. Highest nitrogen fixation in water culture required a strength of culture solution one fifth of that optimal for fixation in sand culture. Fixation in water culture of optimum strength was significantly improved by continuous bubble aeration or by lowering the level of culture solution below the main zone of nodulation. However, if supra-optimal concentrations of solution were used, fixation was markedly inhibited by lowering the solution level, this being associated with an accumulation of high levels of salts on exposed root and nodule surfaces. In – N (minus nitrogen) sand culture continuous waterlogging reduced nitrogen content to 40 per cent of that of non-stressed plants. In nitrate-fed plants waterlogging effects were less severe. Waterlogging decreased nodule tissue production and decreased the specific activity of nitrogenase, as assayed by acetylene reduction. These effects were most marked three or more cm below the sand surface. Watering on alternate days with free drainage at all times yielded maximum fixation in – N sand culture. Regimes increasing the extent of waterlogging or drying out in comparison with this optimum produced increasingly great decreases in nitrogen fixation. For equivalent reductions in total fixation, percentage N in dry matter was consistently lower in waterlogged than in droughted plants suggesting that excess water had the more specific effect on symbiotic activity. Both forms of stress affected particularly the transport of nitrogen from root to shoot.