Microvascular fragments and pure cultures of either retinal microvascular endothelial cells, retinal pericytes or Tenon's capsule fibroblasts were grown on a gas permeable substrate while exposed to varying oxygen concentrations (5, 10, 20, 40 and 95% oxygen). For all three cell types cell proliferation was greatest under the lowest oxygen concentration and decreased as the oxygen concentration increased. The decrease in the proliferative ability of cells exposed to the higher oxygen concentrations could be reversed if the cells were returned to a normoxic environment. Endothelial cells were the most sensitive to changes in oxygen tensions showing a proliferative response after 24 hr exposure as opposed to 48 hr for pericytes and fibroblasts. These results suggest a direct mechanism by which the intra-vitreal and intra-retinal oxygen tension may influence both the development of preretinal new vessels (in response to retinal ischaemia) and also the regression of neovascularization following scatter photocoagulation or vitrectomy.