Previous work has shown that a sward height of 3-5 cm for sheep is the most efficient way of maintaining a balance between new .growth and decayed material. An unreplicated experiment was carrried out during 1985 using 77 cross-bred ewes and their Suffolk x twin lambs to graze a perennial ryegrass sward (Lolium perenne) cv Wendy between 29 April and 22 July (weaning). Animal performance was monitored and grazing behaviour noted. Sward height measurments were taken twice weekly with an HFRO sward stick on a 0.4 ha plot (P) and on the remaining 5.8 ha field (F). Grazing commenced on each area when the sward height reached 4 cm ± 1 and was maintained at this height on P by adjusting animal numbers. Physical data was recorded from a minimum core flock of 5 ewes and 10 lambs. Area F was set stocked at 12.4 ewes/ha. Areas F and P received 198 kg N/ha in total, applied at monthly intervals between early April and July. Net herbage accumulation was recorded at 21 day intervals under exclusion cages on each area.