Intraocular silicone oil has been observed to lessen the incidence of neovascular glaucoma in patients with a high risk of anterior segment neovascularization. In order to test the hypothesis that the presence of silicone oil in the vitreous cavity acts as a diffusion/convection barrier to oxygen and prevents the decrease in anterior chamber oxygen tension that occurs after lensectomy and vitrectomy, we measured anterior chamber oxygen pressure in six cats that had undergone lensectomy-vitrectomy in one eye and lensectomy-vitrectomy with silicone oil placement in the other eye ten to 21 days previously. The anterior chamber oxygen tension was 17.8 +/- 6.9 mm Hg (mean +/- SD) in the eyes that had undergone lensectomy-vitrectomy and 34.8 +/- 3.5 mm Hg in the silicone oil-treated eyes (P less than .001). Silicone oil does appear to act as a diffusion/convection barrier to oxygen and may alter the stimulus for anterior segment neovascularization.