Under lowland conditions there is evidence to indicate that animals consuming ryegrasses bred for elevated water-soluble carbohydrate (WSC) concentrations are able to utilize the protein in their diet more efficiently, resulting in increased live-weight gain, milk production and lower loss of N. The current study evaluated the effects on upland sheep production of grazing two cultivars of perennial ryegrass nominally differing in WSC content. Replicate plots (n=4) of a cultivar bred for elevated levels of WSC (AberDart) and a control cultivar (Fennema) were grazed by ewes and lambs (pre-weaning) and by lambs only (post-weaning). Target surface sward heights of 40 and 60–70 mm were maintained during the pre-weaning and post-weaning periods, respectively. No differences were found in the WSC concentration of the two swards during either the pre-weaning or post-weaning periods. However, cultivar AberDart had a significantly lower fibre concentration during both periods (P<0·05), and a higher digestibility of organic matter in dry matter (DOMD) value during the post-weaning period (P<0·05). There was no cultivar effect of treatment on lamb growth during the pre-weaning period, but lamb live-weight gain was significantly higher for those grazing AberDart than Fennema during the post-weaning period (200 g/d v. 125 g/d; s.e.d.=26·5 g/d; P<0·01), leading to a greater proportion of lambs selected for slaughter by the end of September (0·99 v. 0·70 respectively; s.e.d.=0·062; P<0·05). The improved performance for lambs grazing AberDart relative to those grazing Fennema indicates that advances in plant breeding have the potential to improve the efficiency and profitability of lamb production in the uplands. However, additional research is required to explore the extent to which growing conditions in marginal environments influence the expression of traits.