Components of the N cycle were studied at Hurley, UK, in 1985–87. In grass-clover (Lolium perenne and Trifolium repens) swards, grazed at three intensities, low total inputs of N were associated with low outputs and losses of N. Nevertheless, the flows (intake and excretion) of N through animals were substantial and gave rise, at the higher intensities of grazing, to an acceptably high agricultural output per hectare. This was considered evidence of a fast and efficient recycling of N between plants, animals and soil. The release of N to the environment (as nitrogenous gases and nitrate) was substantially less from the grass–clover swards than from a grass sward fertilized with 420 kg N/ha, and this was at the expense of only 20% loss in production. The mechanisms which might account for the high efficiency of utilization and recycling of N in grass–clover swards are discussed in the context of the balance of the supply of C and N to plant and soil biomasses under grazing. The results confirm that optimizing agricultural output in grass–clover swards has little adverse effect on the environment, but the importance to this end of sustaining a large proportion of N-deficient grass in grass-clover swards is emphasized.