It is with some trepidation that I bring forward this short communication with the object of emphasising the use of a very simple position for radiography of the fourth ventricle.I cannot claim that the position is now a new one, as only last week a paper, which was read by Dr. Lysholm in June 1937, appeared in the BritishJournal ofRadiologyfor May 1938, and some of the illustrations show the position to which I am going to refer. Similar radiographic positions are also reproduced in a paper published in theAmerican Journal of Roentgenologyfor April 1938 by Dr. Robert Stone and Dr. Fritz J. Schulze. There are probably many others using it, but nevertheless I do not think that the importance of this posture is yet generally known. I say this because within the past year or two queries have been received from several sources asking for information as to the technique employed in radiography of the fourth ventricle.Working in collaboration with Mr. A. A. McConnell, F.R.C.S.I., we have used this position in the Richmond Hospital, Dublin, since early in 1935.It is not my intention to discuss ventriculography in general, but to confine my remarks to one aspect of this subject,i.e.,the necessity of adopting a technique which will enable one to visualise the fourth ventricle if it is not occluded and if the aqueduct of Sylvius is patent.