The distribution of serotonin-1 (5-HT1) receptors in the rat brain was studied by light microscopic quantitative autoradiography. Receptors were labeled with [3H]serotonin (5-[3H]HT), 8-hydroxy-2-[N-dipropylamino-3H]tetralin (8-OH-13H]DPAT), [3H]LSD and [3H]mesulergine, and the densities quantified by microdensitometry with the aid of a computer-assisted image analysis system. Competition experiments for 5-[3H]HT binding by several serotonin-l agonists led to the identification of brain areas enriched in each one of the three subtypes of 5-HTl recognition sites already described (5-HT1A, 5-HT1B, 5-HT1C). The existence of these 'selective' areas allowed a detailed pharmacological characterization of these sites to be made in a more precise manner than has been attained in membrane-binding studies. While 5-[3H]HT labeled with nanomolar affinity all the 5-HTl subtypes, the other 3H-labeled ligands labeled selectively 5-HT1A (8-OH-[3H]I)PAT), S-HT1C ([3H]mesulergine) and both of them ([3H]LSD). Very high concentrations of 5-HTI receptors were localized in the choroid plexus, lateroseptal nucleus, globus pallidus and ventral pallidum, dentate gyrus, dorsal subiculum, olivary pretectal nucleus, substantia nigra, reticular and external layer of the entorhinal cortex. The different fields of the hippocampus (CA1-CA4), some nuclei of the amygdaloid complex, the hypothalamic nuclei and the dorsal raphe, among others, also of presented high concentrations of sites. Areas containing intermediate densities of 5-HT1 receptors included the claustrum, olfactory tubercle, accumbens, central grey and lateral cerebellar nucleus. The nucleus caudate-putamen and the cortex, at the different levels rule studied, presented receptor densities ranging from intermediate to low. Finally, in other brain areas reponse, medulla, spinal cord, only low or very low concentrations of 5-HTl receptors were found. From the areas strongly enriched in 5-HTl sites, dentate gyrus and no septal nucleus contained 5-HTlA sites, while globus pallidus, dorsal subiculum, substantia nigra and olivary pretectal nucleus were enriched in 5 HTB. The sites in the choroid plexus, which presented the highest density of receptors in the rat brain, were of the 5-HT1C subtype. The distribution of 5-HTl receptors reported here is discussed in correlation with the distribution of serotoninergic neurons and fibers, the related anatomical pathways and the effects which appear to be mediated by these sites.