Detailed measurements have been made of the pre-retinal partial pressure of oxygen (PO2) in the cat eye to determine the effect of the local geometry of the retinal circulation on vitreal PO2 values. Measurements were made with oxygen sensitive microelectrodes for two ventilation conditions: air and 100% O2 breathing. PO2 gradients, which may extend as far as 800 micron from the internal limiting membrane, have been shown to exist in the vicinity of retinal arteries. Close to a retinal vein there is either a very shallow PO2 gradient or none at all. Coarse vitreal PO2 profiles which were measured out to the "mid vitreous" show that the gradients are very shallow to within 1 mm of the retina. The close agreement between "mid vitreous" PO2 and retinal venous PO2 implies that the retinal venous PO2 sets the PO2 value of the main bulk of the vitreous. Absolute PO2 values indicate that the retinal arterial walls offer easy diffusion for oxygen, and that at 100% O2 breathing almost all the oxygen offloaded from the retinal circulation is from dissolved oxygen. Alterations in the physiological condition of the eye such as those due to changes in the ventilation gas, blood pressure or intraocular pressure may cause relative movement between the microelectrode and the retina. This movement can lead to apparent changes in vitreal PO2 when it occurs in a region in which substantial PO2 gradients exist.