THE exchange of water and other substances between the blood and the contents of the eye is not only an interesting physiologic and biochemical phenomenon but also a vital factor for the maintenance of the intraocular pressure on a certain level. In order to understand better the abnormal variations of the intraocular pressure, one should know as much as possible about the normal mechanism of the exchange of fluid between the blood and the eye, the rate of inflow of the fluid and how this rate may be changed under various conditions. Several attempts have been made to supply the blood with a tracer substance, the presence of which could be demonstrated in the aqueous after having passed through the blood-aqueous barrier. As far back as 1882 Paul Ehrlich1discovered that fluorescein injected into a rabbit's vein will appear in the aqueous after a comparatively short time. However, a