It has been known for some 50 years that birds use photoreceptors in or near the hypothalamus to mediate the photoperiodic responses that control seasonal breeding. So far, however, attempts to identify the photopigment by determining an action spectrum have failed. The problems stem from the selective filtering of light by the tissues surrounding the photoreceptors and the need to deliver defined amounts of light over the days or weeks required to induce a quantitative measure of photostimulation. Here we have developed a technique which produces a quantitative action spectrum for the photoperiodic response in the Japanese quail; the results indicate the presence of a rhodopsin photopigment with a peak sensitivity of approximately 492 nm. The photoreceptors exhibit a level of sensitivity comparable with that of vertebrate visual pigments. We conclude that the brain photoreceptors of birds are based on a rhodopsin/rhodopsin-like photopigment.