Intraocular irrigating solutions used during vitrectomy must be safe for all tissues surrounding the vitreous space. Bicarbonate and glucose were essential to the maintenance of retinal function. In this study the optimum concentration of bicarbonate in the irrigating solution for the retina was determined electrophysiologically with the use of the in vitro eyecup of the rabbit. In 15 to 25 mmole/L of bicarbonate solutions, the electroretinographic (ERG) b wave was well maintained. Higher concentrations of bicarbonate (35 to 45 mmole/L), which have been considered to be advantageous for the integrity of the corneal endothelium, were shown to suppress the retinal activity. These results were also found in in vivo eyecup, as determined with ERG and visual evoked response. Properly performed open-sky vitrectomy, itself, does not cause severe damage to the normal retinal function.