Measurements of electrical potential difference across isolated rabbit corneal endothelium (transendothelial electrical potential difference, TEPD) were used to evaluate the effect of two commonly used commercially prepared intraocular irrigation balanced salt solutions, BSS and BSS +, together with our own standard bicarbonatecontaining control solution (BS). The highest and most durable TEPD values were obtained by using BSS +; TEPD began at 550 μ V, peaked at 600 μ V 1 h later, and then decreased slowly down to some 300 μ V after about 8 h. Gassing BSS+ with an air/5% CO2mixture was optimal; gassing it with air reduced the TEPD values slightly, by about 10%. On the other hand, perfusion with BSS gassed with air had a rapid and marked adverse effect; the TEPD, which began at some 500 μ,V, fell down precipitously to about 50 μ V after 1 hour. In contrast, perfusion with BSS on the endothelial side with the addition of BS to the stromal side, both sides being gassed with an air/5% CO2mixture, maintained the TEPD at levels near those obtained with BSS +. We attribute the adverse effect by BSS to the absence of bicarbonate in it. From these and past results, we conclude that BSS cannot be recommended for intraocular use as a properly balanced saline solution.