There is a high concentration of zinc in ocular tissue, especially the retinachoroids, and the presence of physiological levels of zinc in these tissues seems essential for their normal function. The purpose of our study was to clarify this point using electroretinogram (ERG) recordings and spectrophotometer measurements before and after zinc treatment in a bullfrog's eye. The optimal zinc concentration used in this experiment was 1 µM ZnCl₂, 100 µl/6 ml ringer solution, with the result that the total zinc concentration in the ringer solution was 16.4 nM. To determine the effects of zinc on the retinal function, the changes of ERG parameters, such as threshold, a-, b- and c-waves and absorption spectra were observed before and after zinc treatment. It is noteworthy that high concentrations of zinc are present in the retina and the retinal pigment epithelium. Our results indicate that zinc treatment elevated the dark-adapted ERG threshold, all of the peak amplitudes of ERG were increased and rhodopsin regeneration was accelerated during visual adaptation. In spectral scans, absorbance increment due to zinc treatment was shown over the whole range of spectral scanning (300 nm-750 nm). We believe that zinc, which is abundant in the retina and the retinal pigment epithelium particularly, is an essential factor for the visual process and may be required to improve visual sensitivity during visual adaptation.