The influence of whole, crushed and ground barley and oats on intake, performance and carcass characteristics of lambs was examined. A comparison of diets was conducted with individually-fed male and female lambs in six groups. The study was repeated in three successive years (1995 1997) using the same experimental design. A total of 192 lambs of a mean initial age of 74 days (SD 13.0) and live weight 20.6kg (SD 4.54) were daily fed their respective concentrates, 72g dry matter (DM)/kg metabolic live weight (kgW0.75), for 98 days from weaning to slaughter. Hay was provided ad libitum. The lambs adapted more quickly to diets containing barley than to those containing oats. Compared to oats, the total daily DM intake was higher on barley (1162 vs. 990g DM/lamb, 89 vs. 78g DM/kgW0.75, P <0.001). Hay consumption was significantly lower on oats than on barley (182 vs. 344g DM/day, P <0.001). On oats the lambs experienced energy and protein deficiencies with their energy and protein intakes being 20% below feeding recommendations. On the barley diets the energy and protein requirements of the lambs were satisfied. When processed cereals were offered, there was no increase in the total DM, energy or protein intakes. Processing did not improve the organic matter digestibility of barley or oats. The growth rate (P <0.01), carcass weight (P <0.001) and slaughter percentage (P <0.001) were higher on a barley diet than on oats. A barley diet resulted in more kidney fat (P <0.01) and fattier carcasses. Processing had no positive influence on the live weight gain. On the contrary, lambs receiving crushed or ground barley or oats had a lower final live weight than those receiving whole grains (P <0.001). The utilisation of whole grain was more efficient than that of processed grain. There were no advantages in processing barley and oats for the feeding of lambs.