By means of micro-autoradiographic techniques (3H- resp. 35 S-amino-acids) and histochemical reactions the protein metabolism in the central nervous system (CNS) of 56 Brachydanio rerio and 23 Carassius carassius was investigated with special reference to the axonal flow and a delivery of substances from the nervous tissue into the cerebrospinal fluid (CSF). 1. After intraperitoneal injections of 3H-histine resp. 35 S-methionine and with increasing incorporation periods (7 min to 30 d) a transport of labeled proteins, synthesized in the nerve cell bodies, could be demonstrated from the perikarya to the nerve fiber terminals. 2. After unilateral, intraocular injections of 3H-histidine (incorporation times up to 46 d) the whole visual tract (eye — optic nerve — optic chiasm — optic tectum) of Carassius was labeled by proteins synthesized in the ganglion cells of the retina. 3. The optic nerve fiber terminals (“optic layers”) in the optic tectum of Carassius extend from layer IV to layer VI, that means they have a greater range than it was supposed up to now by means of degeneration studies ( Wawrzyniak , 1962). 4. After intraocular injections of 3H-histidine and beginning with incorporation periods of 3 weeks for the first time a delivery of high molecular and pepsineresp. trypsine-sensitive compounds from the CNS to the CSF could be shown. 5. Experiments with intraocularly injected trypanblue show that higher molecular substances such as proteins in the CNS can probably be transported only intra-axonally. 6. The significance of the delivery of substances from the CNS into the ventricles for the physiological function of the CSF is discussed.