The hypothalamic arcuate nucleus in the male little brown bat Myotis lucifugus lucifugus was studied with the electron microscope. Animals were killed by intracardial perfusion at each season throughout the year so that the arcuate nucleus could be examined for seasonal variations in morphology. Striking seasonal changes in the fine structure of ependymal tanycytes lining the arcuate nucleus were observed. Tanycytes in animals collected in the fall and early winter exhibited pale processes characterized by a scant internal framework of microtubules and fine filaments. These processes, which were found throughout the arcuate nucleus, exhibited simple irregular shapes. In animals collected between January and June, tanycyte processes contained dense accumulations of fine filaments intermingled with microtubules, and projected long attenuated extensions that often formed multilamellar sheets around axodendritic terminals or other neuronal elements. Tanycyte processes of animals collected in July and August were densely packed with microtubules and fine filaments. The processes radiated elaborate multilamellar extensions that encapsulated axons, dendrites and even entire neuronal perikarya. Multilamellar sheets consisted of as many as 10 or 12 closely spaced gyres. The seasonal variations in tanycyte structure are suggestive of astrocyte-like behaviour. These changes are discussed with respect to seasonal changes in hypothalamic neuroendocrine activity.