Copper deficiency induced by a low copper diet in three generations of rats was associated with substantial reductions in the yield of myelin (56%), brain weight (11%), and body weight (43%) in F2 generation rat pups nursed by their own copper-deficient mothers. The composition of the purified myelin was not different from that of controls in the content of individual proteins, lipids, 2',3'-cyclic nucleotide 3'-phosphohydrolase (CNP) activity, or GM1 ganglioside. The major myelin-associated glycoprotein (mGP) was consistently shifted slightly toward higher apparent molecular weight in the copper-deficient animals. Postnatal copper replacement by a foster mother produced a normal yield of myelin per gram of brain tissue, but failed to reverse the deficiency of brain and body growth. After copper replacement in a copper-deficient mother's diet prior to conception, a subsequent litter showed correction of all abnormalities found in her previous litters. The results suggest that copper is essential for myelin formation and general growth during critical periods in development.