The localization of 5-hydroxytryptamine1B receptors in the rat central nervous system was investigated using anti-peptide antibodies that recognize a selective portion of the third intracytoplasmic loop of the receptor protein. At the light microscope level the densest 5-hydroxytryptamine1B receptor-like immunoreactivity was observed in ventral pallidum, globus pallidus, substantia nigra and dorsal subiculum. In addition, moderate immunoreactivity was found in the entopeduncular nucleus, the superficial gray layer of the superior colliculus, the caudate-putamen and the deep nuclei of the cerebellum. This distribution matched perfectly that previously described from radioligand binding studies. At the ultrastructural level, 5-hydroxytryptamine1B receptor-like immunoreactivity was associated with axons and axon terminals in the three areas examined: substantia nigra, globus pallidus and superficial gray layer of the superior colliculus. In all cases, immunostaining was located on the plasma membrane of unmyelinated axon terminals and in the cytoplasm close to the plasmalemma. Synaptic differentiations were never labelled but, in some cases, 5-hydroxytryptamine1B receptor-like immunoreactivity was found in their close vicinity. Injection of kainic acid into the neostriatum resulted in a marked decrease in receptor-like immunoreactivity in the globus pallidus and the substantia nigra, consistent with the location of 5-hydroxytryptamine1B receptors on terminals of striatopallidal and striatonigral fibres, respectively. A reduction in 5-hydroxytryptamine1B receptor-like immunoreactivity was also noted in the superficial gray layer of the superior colliculus after contralateral enucleation, as expected of the location of 5-hydroxytryptamine1B receptors on the terminals of retinocollicular fibres. In both lesion experiments, immunolabelled degenerating terminals were observed in the projection areas. Anterograde labelling experiments coupled with immunocytochemical detection further showed that 5-hydroxytryptamine1B receptors in the substantia nigra are located on axons of striatal neurons. These data provide anatomical support for the idea that 5-hydroxytryptamine1B receptors act as terminal receptors involved in presynaptic regulation of the release of various neurotransmitters, including 5-hydroxytryptamine itself.