An antibody to opsin isolated from rod outer segments of the frog retina was applied in light- and electron-microscopic immunocytochemical studies to the pineal organ of various vertebrates (Cyprinus carpio, Carassius auratus, Rana esculenta, Emys orbicularis, Pseudemys scripta elegans, Lacerta agilis and viridis. Gallus domesticus, Columba livia, Melopsittacus undulatus, Serinus canaria, Taeniopyga punctata, Didelphis virginiana, Erinaceus roumanicus, Myotis myotis, rabbit, rat, cat).