Thirty dairy cows in early lactation were individually fed in stalls on high quality pasture (Lolium peuenne, Dactylis glomerata and Trifoliium repens) and given either formaldehyde-treated casein or untreated casein at 1000 g/day. Nitrogen content and apparent digestibility of herbage nitrogen was 2.8 and 70.4% respectively. Treated casein significantly increased the yield of milk by 13 % and milk protein by 15 % although neither supplement affected milk composition. High-producing cows showed a greater response to formaldehyde-treated casein, with increases in milk yield of 0.5 �0.14 kg per kg increase in level of milk production. Increases in milk synthesis were associated with increased efficiency in utilization of nutrients and not with changes in pasture intake. The results support the hypothesis that formaldehyde-treated casein provided more protein for duodenal digestion and thereby increased the supply of essential amino acids which were limiting milk production. It is concluded that milk synthesis in cows fed solely on high quality pasture in early lactation is limited by the amount of protein absorbed post-ruminally.