In recent years the more physiological aspects of the cerebrospinal fluid have aroused interest, with the result that some progress in elucidating the mechanism of its formation and circulation, and its relationships with the central nervous parenchyma, has been made. The position is still very obscure, however, and it is the purpose of this review to collect the facts from a very scattered literature and to emphasize the gaps, in the hope that they will invite further research. In general, I have used as my starting point in time the years 1955 to 1956, the period when an earlier and more comprehensive review (DAVSON 1956) was going through the press; this has not preeluded the quota- tion of earlier work, however, when this was relevant. In writing this, I have of course been able to benefit by the series of articles in the Ciba Symposium held in London in 1957 and the scholarly reviews of the blood-brain barrier by BAKAY (1956) and DOBBING (1961). I have tried to be comprehensive, but the wide area of the literature over which the published work is scattered means, unquestionably, that I have overlooked many significant contri- 1 butions - 11.