When samples of the ventricle surfaces of human post-mortem brain were examined by scanning- and transmission electron microscopy, varicose nerve fibres could be seen traversing among the cilia and microvilli of ependymal cells. The varicosities contained numerous small electron-lucent vesicles and frequent large electron-dense vesicles, and were usually nonsynaptic but occasionally anchored to the surface by desmosome-like junctions. Supra-ependymal nerve fibres were observed in the lateral ventricles (e.g., n. caudatus), foramen of Monro (stria medullaris), third ventricle (habenula) and floor of the fourth ventricle in brains of the five cases examined. However, only in one of these was a yellow formaldehyde-induced fluorescence observed (on the fourth ventricle floor). Its discrete granular appearance, rapid fading and colour were typical of supra-ependymal 5-HT nerve fibres observed in rat brain. Very recent investigations on serial cryostat sections of rat brain ventricle regions revealed the absence of an immunohistochemical reaction with antisera to substance P, leu- and met-enkephalin and glutamic acid decarboxylase, but the presence of a reaction with 5-HT antiserum. The target for impulse-released 5-HT from this nonsynaptic 5-HT nerve plexus, bathed in cerebrospinal fluid, is not yet known.