Monoamine (noradrenaline and serotonin)-containing nerve fibers distributed in the pineal organ of the dog were studied by means of histochemistry (formaldehyde-induced fluorescence) and immunohistochemistry (peroxidase-antiperoxidase-PAP method) with the use of a serotonin antiserum. With the fluorescence-histochemical technique a dense network of blue-green fluorescent fibers was demonstrated in the pineal organ. Most of these fibers formed a perivascular plexus and their branches penetrated into the intercellular spaces of the parenchymal cells. Since these fibers completely disappeared ten days after bilateral removal of the superior cervical ganglia, it was confirmed that they are noradrenergic post-ganglionic sympathetic nerve fibers. A few yellow-fluorescent fibers were detected in the proximal part of the organ after ganglionectomy. By the use of the PAP method, intensively immunoreactive parenchymal cells and nerve fibers were demonstrated. The distribution pattern of these fibers was similar to that of the fluorescent sympathetic fibers. After almost all immunopositive fibers had been abolished by sympathectomy, some serotonin-containing fibers remained. The latter could be traced back to a system of serotonin fibers in the epithalamic region. These findings suggest that 1) the sympathetic noradrenergic fibers in the pineal organ of the dog take up serotonin which is released from the pinealocytes, and 2) this organ receives a dual monoamine innervation via peripheral noradrenergic and central serotonergic nerve fibers.