S-antigen- and opsin-immunoreactive sites were studied in the pineal organ of toad (Bufo bufo), frog (Rana tigrina), hedgehog (Erinaceus roumanicus) and bat (Myotis myotis) by light microscopic avidin-biotin-peroxidase and electron microscopic immunoglobulin-gold (immunogold), protein A-gold and avidin-biotin-ferritin techniques. The corresponding retinas served as reference tissues. A large number of photoreceptors of toad and frog pineal organ exhibited either strong or weak S-antigen immunoreaction in the outer segments, perikarya and basal processes. A small number of photoreceptors was S-antigen-negative. In general, the intensity of the reaction was stronger in the immunoreactive outer segments of the pineal organ than in those of the rods and certain cones of the retina. In hedgehog and bat, the perikarya and processes of the pinealocytes were either strongly or weakly S-antigen-positive or they lacked immunogold labeling. By use of an antibovine rhodopsin antiserum either strong or weak opsin immunoreactivity was found in the pineal outer segments of toad and frog. A small number of pineal photoreceptors lacked opsin antigenic sites. Double labeling with both antibovine S-antigen and antibovine opsin antisera showed that the opsin immunoreaction was present in the outer segments which also exhibited S-antigen immunoreaction. In the pineal organ of hedgehog and bat, no opsin immunoreaction was observed with the antisera used. It is proposed that in the pineal organ at least two types of photoreceptors are present: one "rod-type" elaborating rhodopsin accompanied by S-antigen and one (or two) "cone-type(s)" using an unknown photopigment(s). Obviously, the different photoreceptors enable the animal to perceive the different wavelengths of the light spectrum.