Diets of different protein content were fed to dairy cows in two experiments of seven months and 12 months duration. Significant differences in the mean concentrations of serum urea, albumin and copper and of blood haemoglobin and packed cell volume were observed between cows receiving the various diets. The greatest differences occurred when cows were in mid-lactation. It was concluded that concentrations of urea in the serum of the cows were reflecting dietary protein intake and that concentrations of albumin, haemoglobin and PCV during lactation were affected by long-term protein status. The possible significance of the results is discussed in relation to previous reports indicating interrelationships between protein and copper metabolism, and in relation to the recommended protein content of diets for cows.