Summary. Two experiments were conducted on feeding grain supplements to dairy cows during summer. In experiment 1, dairy cows in mid–late lactation were offered one of 2 pasture allowances (about 20 and 40 kg DM/cow.day) with and without a supplement of a grain mix (5 kg of a 70 : 30 mixture of barley and lupins). This experiment was conducted twice over 2 consecutive summer–autumn periods (years 1 and 2). Milk yields were higher at the high pasture allowance and with grain feeding. The marginal response to grain feeding (kg milk/kg grain DM consumed) was 0.7 at both pasture allowances in year 1, and 1.1 and 0.8 at the low and high pasture allowance, respectively, in year 2. Diet had no significant effect on milk composition. Pasture intake was reduced by supplementary feeding and the level of substitution depended on pasture allowance. The alkane-based and sward sampling techniques were used to estimate pasture intake and diet digestibility. Estimates of pasture substitution, based on measurements made with the alkane technique, were 0.3 and 0.6 kg DM reduction in pasture/kg DM grain consumed at the low and high pasture allowance, respectively, in both years. In year 1, the pasture substitution measured with the pasture difference method was 1 kg pasture DM/kg grain DM at both pasture allowances. In year 2, estimates of substitution calculated using the pasture difference method were 0.2 and 0.5 kg pasture DM/kg grain DM at the low and high pasture allowance respectively. In the second experiment (year 3), the effects of level of feeding barley on pasture intake, diet digestibility and milk production were measured during summer. The 4 levels of grain consumed were 0, 1.8, 3.4 and 6.7 kg barley DM/cow. day. Higher grain intake resulted in higher milk yield, higher protein concentration and yields of fat, protein and lactose. Milk yield responses per kilogram of barley diminished from 1.6 at the low barley intake to 0.8 kg milk/kg barley DM at the highest barley intake. It is suggested that higher levels of substitution and a potentially less efficient use of grain contributed to the poorer production responses at higher levels of grain feeding.