The main developmental events in the subfornical organ take place between 17 fetal days (fd) and 5 days post natum (dpn) at which time it possesses most of its mature fine structural characteristics. The surface regional characteristics of ependymal cells differentiate primarily during this time as well, while the ependymal cellular fine structure, shape and relationship with neurons and the vascularity are well established prior to birth. Undifferentiated neurons contain glycogen prior to 19 fd and then differentiate by developing processes and organelles characteristic of neurons. By 5 dpn, the various types of neurons found in the mature subfornical organ are all present, except for giant vacuolated cells. Synapses containing only electron-lucent vesicles are first present at 20 fd, those containing additional electron-dense vesicles at 3 dpn. Microglial cells are first identifiable at 17 fd, and the first protoplasmic astrocytes are recognizable at 21 fd, while fibrous astrocytes are not detectable prior to 7 dpn. By 5 dpn, the cytological elements of the subfornical organ are all in place, and further developmental changes leading to adult fine structural characteristics by 30 dpn are essentially quantitative in nature.