The pineal organs of the adult domestic mallard, fowl and pigeon were investigated by means of neurohistological techniques and electron microscopy. The application of opsin immunocytochemistry to the pineal organ of the domestic mallard reveals antigenic sites in the outer segments of the pinealocytes. Avian pinealocytes may establish synaptic contacts to intrapineal neurons via complex neuropil formations. Bundles of nerve fibers resembling the pineal tract of poikilothermic vertebrates were found in the pineal stalk. These findings strengthen the view that the avian pineal organ has not lost its photoreceptive capacity.