Studies on the zonal distribution of juxtaglomerular granules in mice ranging in age from three days to adulthood have revealed that the adult distribution, in which there are significantly more granules in the superficial zone of the cortex than in the juxtameduUary zone, is achieved at about the third week of extra-uterine life. This correlates well with the maturation of the long loops of Henle in rats and the onset of maximal urinary concentrating ability in humans, and suggests that variations in the juxtaglomerular index at different ages may bear a relation to functional development.