The secretion of the subcommissural organ (SCO) is synthesized in the intrinsic cells of that structure, a neuroendocrine gland. The organelles involved in the synthesis of this secretion are rough-surfaced endoplasmic reticulum (RER) and the Golgi apparatus. It is still uncertain whether and to what extent the latter participates in the synthesis. In highly active secretory ependymal cells of the SCO, the Golgi apparatus exhibits distinct signs of intense activity. This suggests that the Golgi apparatus is involved in the preparation of the secretion, even if this is difficult to prove electron microscopically. The secretion is discharged from the optical part of the SCO-cells into the cerebrospinal fluid (CSF) of the IIIrd ventricle, where Reissner's fibre is formed. Different phases of release of the content of the secretory granula into the IIIrd ventricle are described; our findings are in agreement with previously published observations. In 2 cases delicately granulated and moderately electron-dense material was found in circumscriptly dilated vaults of the intercellular spaces. Release of secretory material into intercellular spaces appears to be possible, but is evidently infrequent. Regular occurrence of a basal (peripheral) discharge into the hypendymal capillaries is not unanimously agreed upon as yet. In this paper, a passage of granula is described from the cytoplasm of the end feet of SCO cells through the walls of the capillaries into the systemic circulation. This provides structural evidence that secretory material synthesized in the SCO is released into the capillaries. These observations must ultimately be confirmed with the aid of ultracytochemical methods, particularly using labelled material.