Aminergic brain structures have been investigated by means of light microscopical autoradiography after injection of the tritiated catecholamines noradrenaline and dopamine and the indoleamine (or tryptamine) serotonin into the brain cavity of frogs of the Rana esculenta complex. These amines are fairly specifically taken up by catecholaminergic and serotoninergic neurons, respectively, which are located in structures like the catecholaminergic preoptic recess organ; the mixed catecholaminergic-serotoninergic paraventricular organ/nucleus infundibularis-complex and nucleus reticularis mesencephali; the telencephalic septal and striatal areas and the tectum opticum, which contain many catecholaminergic axon terminals; the habenular area, which contains serotoninergic axon terminals. The autoradiographical data on the location and the nature of these aminergic brain structures agree well with the mainly fluorescence microscopical and immunocytochemical data from the literature. The autoradiographical detection method can be combined at the light and the electron microscopical level with other histological, histochemical, or immunohistochemical techniques in one and the same preparation, and the results of the different treatments may eventually be made visible simultaneously.