We studied the toxic effects of sulfur hexafluoride and perfluoropropane in comparison with air, balanced salt solution, hyaluronate sodium, and aqueous humor in a rabbit model. Sixty normal pigmented rabbits were studied during a period of 4 weeks. The variables studied were slit-lamp biomicroscopic examination of the anterior segment, intraocular pressure as measured by pneumotonometry, corneal thickness and endothelial cell count as measured by specular microscopy, lens opacity by Scheimpflug photography, and light and transmission electron microscopy. All three gases were more toxic to the cornea and lens than were balanced salt solution, hyaluronate, and aqueous humor. However, 15% perfluoropropane and 50% sulfur hexafluoride were no more toxic to the eye than air was. Therefore, this study suggests that nonexpansile mixtures of perfluoropropane and sulfur hexafluoride may be beneficial and relatively safe in re-forming persistently flat anterior chambers in situations where the use of air is being considered.