The ultrastructure of pituitary of female eels at seaward migrating periods was investigated with the electron microscope. Neurohypophysis is mainly composed of neurosecretory fibres, pituicytes and capillaries. A few neurosecretory fibres were myelinated, but most were not. Some multilamellate bodies were present in neurohypophysis. Two types of neurosecretory axons are found in the neurohypophysis, type A fibres containing elementary neurosecretory granules with diameter of 1250—1750Å, type B fibres containing granulated vesicles with diameter of 450—1000Å. These fibres end on a basement membrane adjacent to the endocrine cell of adenohypophysis. So, the endocrine cells generally do not contact directly with the neurosecretory fibre. However, some of the neurosecretory fibres make direct synaptic contact with pituicytes. Six different types of endocrine cells were identified in the adenohypophysis by the ultrastructural characteristic and the size of the secretory vesicles they contain, i. e., prolactin cell, corticotroph, somatotroph, thyrotroph, gonadotropyh and the endocrine cell in meta-adenohypophysis. Prolactin cells, corticotrophs and thyrotrophs exhibited an active secretory function, whereas gonadotrophs did not show a functional activation.