This chapter offers additional morphological and functional data suggesting that ependyma of the median eminence are capable of recovering thyrotrophin-releasing factor (TRF) from cerebrospinal fluid (CSF) and delivering it to the portal vessels. This capability is necessarily a fundamental requisite to a hypothesis in which the CSF is a part of the route of delivery of releasing factors from brain to the anterior pituitary. A major unresolved problem is the identity and location of those specialized parvicellular neurones in the central nervous system (CNS) responsible for secretion of the releasing factors. On the assumption that the parvicellular, releasing factor (RF)-producing neurosecretory system projects directly to the median eminence, much work is done to identify morphological correlates of their neuroendocrine activity in terminals and axonal inclusions of the palisade layer. Infusion of TRF into the lateral ventricle of the rat results in plasma thyrotrophin (TSH) responses whose time course is similar to that following intravenous infusion of TRF. It is suggested that the ependyma of median eminence are capable of transporting TRF from cerebrospinal fluid to the pituitary portal capillaries; thyroxine, and possibly catecholamines, influence the rate of this transport.