The development of intraventricular axons in the infundibular recess of the young rat was investigated by correlative scanning and transmission electron microscopy (SEM-TEM). From the fourth through the fifteenth day of life such axons increase steadily in number. During subsequent weeks their number gradually decreases. In animals given monosodium glutamate on the fourth postnatal day there is widespread neuronal necrosis in the arcuate nucleus, and the development of intraventricular axons is greatly reduced. These findings suggest that the axons originate from the neurons of the arcuate nucleus.