In order to localize the cells expressing 5-HT1A receptors in the rat brain, we used in situ hybridization histochemistry to visualize the distribution of the mRNA coding for 5-HT1A receptors. Oligonucleotides derived from different parts of the coding region of the rat 5-HT1A receptor gene were used as hybridization probes. 5-HT1A binding sites were visualized on consecutive sections by receptor autoradiography using 3H-8-hydroxy-2-(di-n-propylamino)tetralin as ligand. The highest levels of hybridization were observed in the dorsal raphe nucleus, septum, hippocampus, entorhinal cortex, and interpeduncular nucleus. Positive hybridization signals were also present in other areas, such as the olfactory bulb; cerebral cortex; some thalamic and hypothalamic nuclei; several nuclei of the brainstem, including all the remaining raphe nuclei, nucleus of the solitary tract, and nucleus of the spinal tract of the trigeminus; and the dorsal horn of the spinal cord. The distribution and abundance of 5-HT1A receptor mRNA in different rat brain areas generally correlate with those of the binding sites, suggesting that 5-HT1A receptors are predominantly somatodendritic receptors.