Light-induced changes in extracellular H+ concentration (delta pH0) were studied with intraretinal H(+)-sensitive double-barreled microelectrodes in frog eyecup and isolated retina preparations. The most prominent delta pH0 were found in the inner plexiform layer, as pH increases (alkalinizations) at light onset and offset. With a small-spot stimulus (0.3 mm dia.), 30 sec in duration, the delta pH0 were relatively small (0.03 pH units), and long lasting (peak at 25-30 sec). They were enhanced by flicker (0.3 Hz). Depth profiles paralleled those of the field potentials (PNR/M-wave), the ON delta pH0 peaking 40 microns more proximal than the OFF response. The delta pH0 exhibited surround antagonism, which was blocked by tetrodotoxin (TTX), indicating an independence from action potentials. The mechanism for these pH increases in proximal retina is not yet understood. In the subretinal space diffuse retinal illumination produced a small pH increase, consistent with a presumed decrease in photoreceptor lactate production. Inhibition of carbonic anhydrase (CA) with acetazolamide or methazolamide increased both the proximal and distal retinal delta pH0, suggesting that CA is involved in buffering retinal pH.