The growth and chemical development of the brain and spinal cord of the pig have been studied between the 52nd day of gestation and 3 years of age. The brain increased in weight faster than the spinal cord. Its most rapid period of growth lasted from about 50 days before birth to about 40 days afterwards and was made up of an early period characterized by DNA-P deposition and a later one of lipid deposition. The percentage of water in each part of the central nervous system fell during development and the concentrations of total N, phospholipid-P and cholesterol rose. The concentration of DNA-P fell gradually over the period studied in the brain as a whole, but in the cerebellum and brain stem the fall was preceded by a rise to a peak concentration at about 95 days of gestation. The cerebellum was the only part of the central nervous system examined in which the absolute amount of DNA-P attained a mature value before three years of age. The spinal cord had no early growth spurt and for this reason there was no time when the absolute amounts of cholesterol and phospholipid-P were rising conspicuously rapidly. There was, however, a period when the concentration of cholesterol increased most rapidly and this coincided with the one in other parts of the central nervous system. The bearing of these results on the effects of stress on the development of the central nervous system is discussed.