An attempt was made to obtain normal values of the sizes of glomeruli in the fetus and child. The kidneys of 117 children from 12 weeks gestation to 5 years of age were measured and the difference in size between the juxta-arcuate, mid-cortical and superficial glomeruli was examined. Juxta-arcuate and mid-cortical glomeruli showed an initial decrease in size from 12 to 20 weeks gestation. This was not seen in the most superficial glomeruli. After the initial decrease, the juxta-arcuate and superficial glomeruli remained at the same size until birth. The superficial glomeruli remained the same size from 12 weeks gestation to term. There was an immediate increase in size after birth in all three groups which slowed down after 2 years, when all three groups became the same size. The changes in size of the juxta-arcuate and mid-cortical glomeruli may be explained by functional demand.