Abstract 1. 1. Pups were subjected, from birth, to protein undernutrition by feeding the lactating dams 8% casein (CS) or 8% soy protein (SP) diet up to weaning; the weanlings were fed the same diets until 6 weeks of age. 2. 2. At 3 and 6 weeks of age, myelin was isolated from the brains and characterized. The quantities of myelin and its content of cholesterol, galactolipids and phospholipids, were significantly depressed in the 8% CS and 8% SP groups but not when soy protein was fed at the same level as casein (25%) in the control. 3. 3. Furthermore, the severity of the deficits in myelination showed a differential pattern depending on the type of dietary protein fed. 4. 4. At weaning, the deficits with the 8% SP diet were 1.5–2.0-times greater than with the corresponding casein diet. 5. 5. A more pronounced retardation in the initiation, progression and capacity of myelination in postnatal soy protein undernutrition was indicated.