Pathological observations on brain damage in baby rats with experimental copper deficiency were carried out and the following observations were made: 1. The cerebrum were decreased in size and weight, the cortex became thinner and the corpus striatum atrophic. 2. Vacuolar degeneration and swelling of the mitochondria in the extrapyramidal system within the corpus striatum resulted in axon damage and disintegration of the myelin sheath. 3. Proliferation of endoplasmic reticulum in the dark neurons, swelling and rupture of mitochondria in the light neurons, which can result in necrosis of both types of neurons. The damage due to copper deficiency could not be reversed by copper supplements in diet.