The rat retina and the different brain regions contain membranes sites that bind L-lysine in the nanomolar range. These binding sites undergo changes in different experimental conditions, thus: intraocular injection of kainic acid induces a reduction of the density of L-lysine binding sites, D,L-alpha-aminoadipic acid injected into the eye enhances both kinetic parameters (Bmax and Kd) of L-[3H]lysine binding sites, the intraperitoneal injection of iodoacetic acid decreases the sensitivity for its ligand binding sites, and the exposure to darkness of the rats reduces L-[3H]lysine binding in the retina, thalamus, hypothalamus and superior colliculus, but not in the occipital cortex; such a decrease appears to be characterized, at least in the retina, by a lower sensitivity of the binding sites for L-lysine after the exposure to darkness. The results show that L-lysine binding sites are located on kainic acid-sensitive cells and can be involved in the physiological mechanism of vision.