Activation of 5HT1A receptors produces many different physiologic responses, which may be due to their localization on diverse cells in the brain. A 5-HT1A receptor antipeptide (aa170-186) antibody was produced that showed both high titer for peptide binding and immunocytochemical staining. Studies performed in perfusion-fixed brain tissue showed immunoreactive neurons, glial, and ependymal cells in the rat, mouse, cat, and monkey. Results from our studies of Macaca fascicularis brains are presented. We observed two main neuronal labeling patterns in the primate brain: (1) A general, diffuse somatodendritic distribution of 5-HT1A receptor immunoreactivity is seen in the raphe nuclei where the dendritic shaft, its branches and spines, and the entire perikaryon are immunolabeled. This pattern is also observed in the nucleus locus coeruleus, in scattered large brainstem reticular neurons, and in dentate gyrus hilar interneurons. (2) A discrete localization of 5-HT1A receptor immunoreactivity on the initial axon segment (axon hillock) is noted in pyramidal neurons of layer III and V of cerebral cortex, Cornu Ammonus (1-4) of the hippocampus, and in most brainstem and cervical spinal cord motoneurons. In addition to neuronal labeling, 5-HT1A receptor immunoreactivity is seen in the cell body and processes of astrocytes, and other nonneuronal cells. This pattern is particularly evident in the white matter of cerebral cortex and spinal cord, the pontine nuclei, the brainstem tectum, and the hilus of the dentate gyrus. The clinical implications of 5-HT1A cellular localization are briefly discussed.