Copper deficiency was investigated in the guinea pig because this species resembles the lamb in that it undergoes considerable myelination in utero. When a semipurified pelleted ration containing 0.5–0.7 ppm copper was provided young female guinea pigs during growth and gestation a high incidence of ataxia, gross brain abnormalities and aneurisms was observed among the progeny. Agenesis of the cerebellum was one manifestation of defective nervous tissue development. Other animals showed gross changes only in the cerebral cortex tissue which was soft and translucent. Liver stores of copper proved to be within the range of values reported by others as characteristic of copper depletion in lambs known to have swayback.