Physiological, pharmacological and radioautographic binding studies have suggested the presence of the 5-HT1A autoreceptors on midbrain serotoninergic neurons. The recent production of specific anti-rat 5-HT1A receptor antibodies in rabbits injected with a synthetic peptide has provided a tool to examine this problem directly. Using the immunoperoxidase method to localize the receptor protein, neurons of all the sizes and forms characterizing the neuronal populations in the dorsal and median raphe nuclei were stained. Reaction product was distributed along the neuronal surface, outlining the contours of perikarya and dendrites in a continuous but uneven manner. Intracellular staining was scarce and confined to the perinuclear region. Double immunohistochemical staining using the anti-5-HT1A receptor antibodies and an anti-serotonin (5-HT) antiserum showed that all the 5-HT1A receptor immunoreactive neurons in the dorsal raphe, and the vast majority of them in the median raphe, are serotoninergic neurons. These data provide the first direct demonstration of the existence of 5-HT1A autoreceptors on the perikarya and dendrites of serotoninergic neurons in the anterior raphe nuclei.