During the period 1974-77 six blocks of a perennial ryegrass/white clover sward were each divided into two for rotational paddock-grazing and silage cutting in alternate years. For each use an annual total of 360 kg nitrogen per ha was applied. Six livestock treatments of cattle only or cattle and sheep were grazed on the blocks. The stocking rates per ha and combinations of cattle (yearling steers) and sheep (ewes nursing twin lambs) respectively were low, 7·5 + 0; medium, 10 + 0, 7·5 + 5, and 5+10; and high, 7·5 + 10 and 5 + 15. The annual quantities of herbage organic matter accumulated (6·6 to 7·2 t/ha) and consumed (5·6 to 6·7 t/ha) did not differ markedly according to grazing treatment during an individual year, nor was there an effect on organic matter accumulation in subsequent conservation years (11·3 to 12·0 t/ha). Between-year effects were significant. As judged by persistence of sown species there was no evidence of sward deterioration over the 4 years. Total live-weight gain per ha increased with increasing stocking rate but with a decreasing increment. The ranges were 106 to 1·42 t/ha within treatments, 102 to 1/45 t/ha within years and 0·97 to 1·8t/ha within treatments × years. Individual gains of cattle (0·67 to 0·88 kg/day) and lambs (0·20 to 0·27 kg/day), and the proportion of lambs ready for slaughter (0·53 to 0·97), were lowest at the highest stocking rate. Mixed grazing compared with cattle-only led to improved cattle gains and improved total gains per ha. The benefits of mixed grazing for a species tended to increase as the proportion of that species in the mix decreased.