2 trials were conducted with female rats to investigate in which way the level of dietary Cu influenced the level of Cu concentrations in rat milk. The trials were made to establish whether data on the Cu content of milk might be used as indication of the level and state of Cu supply to the animal. In the period of colostrum formation the Cu content of the milk was shown to be clearly dependent on the level of dietary Cu. In later periods of lactation Cu concentrations in the milk (from 2 mg of Cu per kg of diet) were found to be unaffected by the Cu content of the diet. Constant Cu concentrations of between 2.5 to 3.0 mg per kg of milk were noted in this period. It was only in cases of extreme Cu deficiency (0.5 mg/kg of diet) that soon after parturition the Cu values decreased to and remained at a very low level of 1 mg per kg of milk. Generally, colostrum contained 4-6 times more Cu than ordinary milk (in all groups).