After 21-day-old weanling rats were maintained on diets deficient in riboflavin the weights of their brains were 19.8% less than those of rats on control diets. In riboflavin deficiency, the myelin lipids, cerebrosides, and sphingomyelin, as well as phosphatidylethanolamine, a significant component of the myelin membrane, were considerably reduced in proportion. It is considered that riboflavin plays some role in the metabolism of essential fatty acids in brain lipids and the pathological effect of its deficiency is similar to that of essential fatty acid deficiency, causing a fast impairment to brain development and maturation.