The role of the venous pressure in the maintenance of intraocular pressure has been the subject of many reports. Duke-Elder5,6determined venous pressure in the Circle of Hovius of dogs, by the micropipette technique, and found a close parallelism between it and the intraocular pressure. In man, Linner found the episcleral venous pressure to be relatively constant in normal and glaucomatous eyes and apparently unrelated to intraocular pressure.11,12,13Some authors have reported, however, that this pressure can vary, such as with diurnal variations on intraocular pressure and pilocarpine administration.1,3,4,17,18Linner14reports that the pressure is unchanged after acetazolamide (Diamox) administration. The venous outflow systems of man, dog, and cat differ in one important aspect. In man there are no substantial communications of the anterior ciliary venous circulation with that of the vortex bed, while in the other two species such communications exist. Data on the effects