Different localizations of secretory material are noted in adult and fetal subcommissural organ (SCO) in light microscopy. At the electron microscope level, the secretory ependymocytes reveal frequent associations among mitochondria and ribosomes of the endoplasmic reticulum (ER). In the SCO ependymocytes of the adult rat, the relationship between mitochondria and ribosomes of the ER is observed in the subgolgian zone, the ER cisternal profiles are smooth except where they face the mitochondria. Here, a constant interval of 40-45 nm separates the ribosome-coated ER membrane from the external membrane of the mitochondria. This association evidences a functional cooperation between mitochondria and ER, at least in some phases of the synthesis of the organ's gliosecretory material. By contrast, in the fetus (17-21 fetal day), the synthetic apparatus displays an entirely granular ER. The secretory products are stored as flocculent material which fills the ER cisternae. In the apical zone of the ependymocytes, as the membrane of the dense secretory granules fuses with the apical plasmalemma, the granules release their contents into the ventricular cavity. A possible link between the releasing process and the coated vesicles is discussed.