• The permeability of the blood-retinal barrier to infusion of fluorescent tracers was examined in tissue sections of frozen eyes from conscious rats that had breathed either air (control) or 25% carbon dioxide in air for one hour. For all animals the blood-retinal barrier remained impermeable to either carboxyfluorescein or Evans blue, indicating a functionally intact tight junctional barrier during hypercapnia. However, in hypercapnic animals, fluorescein penetrated into the neural retina, mainly via the pigment epithelium. Fluorescein also passed through the pigment epithelium in rats with metabolic acidosis induced by intravenous infusion of ammonium chloride, which lowered arterial blood pH without raising the Paco2 or BP. The results indicate an increased permeability of fluorescein through the cell membranes of the pigment epithelium during respiratory and metabolic acidosis. The effect on fluorescein may be due to a pH-dependent increase in the plasma concentration of the associated, more lipid-soluble form of this weak acid.