During the process of light-adaptation, a part of retinal S-antigen (“48-kDa protein”) is bound to the photoreceptor cell membranes. This fraction can be isolated by first extracting the soluble S-antigen with isotonic buffer and subsequently extracting the bound S-antigen with detergent. In this way we found that light-adaptation to 250 1x or more induces a maximum binding of 62% of total S-antigen within 2 minutes in rat retina in vivo. At low light intensity (50 1x) this process lasts 15 minutes, while at 5 1x only 30% of S-antigen is bound. Presumably the number of available phosphorylated (bleached) rhodopsin molecules is the limiting factor in time and quantity. Dark-adaptation causes an initial rapid release of S-antigen during the first 5 minutes, but it takes more than 2 hours to reach the minimum level of about 10% bound S-antigen. The rates of binding of S-antigen in the light and of release of S-antigen in the dark are compared to other phenomena of light and dark adaptation.