A quantitative study of the morphogenesis of granule and pyramidal cells was performed on Golgi-Cox preparations of the developing hippocampus of normal and hypothyroid rats, and hypothyroid rats given replacement thyroxine treatment. In the normal hippocampus, the volume of the cell body and the number of branching points on the apical and on the basal dendrites of pyramidal cells increased between 6 and 10 days after birth. The pyramidal cells of Ammon's horn showed a gradation from area CA1 to area CA4 of progressive differentiation. In thyroid-deficient rats, the arborization of the dendritic field of both granule and pyramidal cells was impaired, and for pyramidal cells the extent of the impairment depended on the position of the cells in the Ammon's horn, the cells of CA3-4 areas being the most affected. Treatment of hypothyroid rats with a physiological dose of thyroxine restored some of the morphological defects to normal, but others were altered beyond control levels, indicating that thyroid hormone differentially controls not only the measured indices of maturation but also that its influence depends on the position of the pyramidal cells. The observations were consistent with the concept that thyroid hormone is important in the establishment of the CA1 to CA4 gradient of pyramidal cell differentiation and in the development of the spatiotemporal relationship between pyramidal and granule cells of the hippocampus.