Pre-retinal partial pressure of oxygen (PO2) profiles have been measured for the first time in the isolated arterially perfused eye of the cat. The perfusate, fed to the eye under a constant pressure head, contained no haemoglobin or haemoglobin analogue so that the only source of oxygen was in the dissolved form. Maximum oxygen supply is provided by raising perfusate PO2by equilibration with 100% O2. Periodic measurements of the DC corneal electroretinogram were used as a check on preparation viability. Using oxygen sensitive microelectrodes, oxygen tension profiles were measured in the vicinity of retinal vessels and also in vessel free areas, to determine the effect of the retinal circulation on the local oxygen environment of the vitreous. Results showed that large PO2gradients exist near retinal arteries whereas more than 1mm away from the retina the PO2gradient is very shallow. These results are in agreement with those obtained for the in vivo cat eye where the animal is breathed on 100% oxygen. Thus it can be concluded that the isolated perfused eye behaves similarly to the in vivo eye with respect to oxygen distribution and function.