Trials with graded Cu supplementation were carried out to establish the minimum and optimum Cu demands of pregnant, lactating and non-pregnants rats. The rats were investigated for the rate of live weight gains of the dams and young rats, for the number of stillbirths and, in particular, for the Cu status in the liver, body and blood serum of the animals and for coeruloplasmin activity. All Cu values obtained from the organs investigated had reached a saturation level at 8 mug Cu/g diet with the exception of the values for body Cu found in the dams that were killed on the day of delivery. The same results were obtained, for the above criteria, with lactating rats. So, it may be concluded that pregnant and lactating rats receive an optimum supply of Cu when they are fed a starch + casein diet containing 8 ppm Cu. For non-pregnant female rats, the limits that might be regarded as adequate were even below the 8 ppm level of dietary Cu. The saturation level for Cu was also found to be dependent, in regard of the criteria investigated, on the physiological state of the animals. It was found that, on the day of parturition, the level of Cu concentration and coeruloplasmin activity in the serum of the dams and the Cu content in the liver of the new-born rats were the best indicators for the state of Cu supply to the dams. However, after a sucking period of 17 days, the level of total Cu content in the dams was found to respond very sensitively to changes in the Cu supply.