Groups of 3 preterm (duration of gestation 22 days), 3 term (23 days), and 3 postterm (24 days) rats have been fed together by 1 mother rat. The postnatal growth of liver, brain, heart and kidney has been studied up to the 30th day of life. The postnatal loss of water shows good correlation with the maturation of each organ. The postnatal loss of water is marked in brain and kidneys, but only slight in liver and heart. The pronounced growth spurt of the brain at the about 10th day of life is probably due to the opening of the eyes. The growth spurt of the liver at the 15th day may be due to the supplementary food at the time of weaning. The effect of intrauterine undernutrition in postterm rats is most strikingly demonstrated in the liver. Postnatal undernutrition in preterm rats causes a retardation of growth of the liver and kidneys.