A knowledge of the chemical composition of the dairy cow is Important in interpreting the interacting influences of body reserves, diet and feed intake on milk production. An investigation has designed to obtain information from a representative sample of Friesian/Holstein cows at different stages of lactation, different ages, and of different yield potential. This information is expensive to collect due to the costs Involved in slaughtering and analysing dairy cows, especially if they are high yielding cows at peak production. The animals used in the investigation have therefore all been Planned culls with sixteen in the later stages of lactation with only three killed in early lactation. It is intended to continue this investigation for several years to obtain a comprehensive data set on the chemical composition of the dairy cow.