After treatment of the isolated retina of a fish (Centropomus) with ammonia (NH(3)), the electrical responses to light are confined to the receptor layer. These receptor responses are fast in light adaptation and very slow in dark adaptation. The light-adapted responses have thresholds 3 to 4 logarithmic units of light intensity higher than the dark- adapted ones. Both kinds of responses are linearly related to light intensity up to the appearance of saturation. Interactions between receptors and adjacent glia cells appear to be involved in adaptation.