Measurements of vitreal oxygen tension have been made for the first time in the streptozotocin-induced diabetic rat eye. A total of 36 Sprague Dawley rats were divided into a control (n = 18) and streptozotocin injected group (n = 18), and after 5-6 weeks of established hyperglycaemia, an acute experiment was performed in which vitreal oxygen tension profiles were determined with oxygen sensitive microelectrodes. The control rats had significant oxygen tension gradients in the vitreous close to retinal arteries with relatively flat oxygen tension profiles close to retinal veins and intermediate regions. All control rats had a substantial arteriovenous oxygen tension difference when measurements were made on retinal arteries and veins. In contrast the oxygen tension profiles measured in the vitreous of streptozotocin rats showed markedly reduced oxygen gradients in the vicinity of retinal arteries and a smaller arteriovenous oxygen tension difference. In both groups of rats, for distances of 500 microns and greater from the retina (mid vitreous) a plateau oxygen tension value was observed. No significant difference was found in this mean mid vitreous value between the control rats and diabetic rats under the same systemic conditions. We conclude that there are significant changes in oxygen tension near retinal arteries in streptozotocin-induced diabetes before any histopathological changes are evident.