One of the main purpose in improving our knowledge of nutrient requrements and supplementation to ruminants is to optimise resource utilization and minimize production costs. During the last years, our knowledge of dietary protein evaluation for diary cows has increased, and in Norway a new semi-mechanistic feed evaluation system has been developed, which has improved the efficiency of protein feeding in dairy cows. Further im provement in prediction of dietary effects on milk protein production requires an improvement of our basic understanding of factors affecting intermediary metabolism of amino acids and glucogenic substrates. Important areas of future research are metaboli sm of nutrients in the gastrointestinal tissues and the liver, because it plays a dominant role in nutrient delivery to the mammary gland and in determining the ultimate quantify and patterns of metabolites available for milk synthesis. Moreover, focus al so should be placed on the nutrient supply x genotype interaction since it is suggested that high genetic merit cows are more efficient because they partition the available nutrients differently from low genetic merit cows. In this project we will study milk protein response to abomasal or intravascular infusion of glucose or amino acids in cows with different genetic merit.