The ultrastructure of the neural lobe of rats from birth up to the 340th day of life was studied. The distinguishing feature during the first week of post-natal life were axons consisting almost entirely of neurotubules, with only very few neurosecretory granules. Nerve endings contained scattered synaptic vesicles. Pituicytes had processes that were either thick or thin and long. The latter floated freely about an ample intercellular space. The intercellular space grew smaller during the following weeks. The neurosecretory granules increased in number, sometimes filling up the whole section of the axon. Dilated axons, from which the Herring bodies originate, began to appear, and the pituicytes also lost their thin processes. Between the 30th and 150th day of life, the neural lobe acquired the usual features of an adult gland. By the end of the first year, the connective tissue had developed greatly. The amount of axons containing lysosomes had increased. Furthermore, it can be said that the neurosecretory axon of the new-born rat is tubular and has no dilations. The neurosecretory granules are scarce. After the 21st day, the axonal dilations filled up with neurosecretory granules begin to grow, making the axon look like a string of beads.