The choroid plexus as a source of cerebrospinal fluid was suggested by Faivre1in 1854, yet, while the existence of this function was generally suspected, it was many years before convincing evidence became available. In the last few decades, however, contributions from the fields of chemistry, anatomy, embryology and physiology, as well as observations at the operating table, have added much to the knowledge of this fluid system. While no single observation has been of a conclusive nature, the summation of available evidence led to the general acceptance of the theory that the cerebrospinal fluid is a product of the choroid plexus and that it has a definite circulation, the movement being in the direction of the subarachnoid space from whence it is resorbed into the blood stream, the mechanism of resorption remaining in doubt. The earlier investigators considered the fluid to be a true secretory product, but more