It has been suggested that the tunica vasculosa lentis of newborn mice undergoes vasoproliferation after oxygen exposure and might be used as an experimental model for intraocular neovascularization. A scanning electron microscopic technique which provides visualization of the entire hyaloid vascular system was used to study its response in mice exposed to various gas mixtures (70% O2/30% N2, 10% CO2/90% air, 10% CO2/70% O2/20% N2). Marked differences were found between hyaloid regression in gas exposed mice compared to air control mice, but no evidence of neovascularization was found in any group. The data support the possibility that changes in the hyaloid may be the consequence of the hydrostatic coupling which exists between this atrophic, possibly flow-resistant vasculature and the developing retinal vasculature whose blood flow characteristics are significantly altered by exposure to CO2 and O2.