The regional distribution of the mRNA encoding the serotonin 5-HT1B receptor was studied in the central nervous system of the rat by in situ hybridization histochemistry and Northern blot analysis. A 180 base pair probe, corresponding to a highly selective portion of the third intracellular loop of the rat 5-HT1B receptor, was used. In most regions, a single 5 kb message was found by Northern blot analysis. However, two additional bands (2.5 and 4 kb) were detected in the striatum. The rank order of 5-HT1B mRNA abundance was striatum >> septum = ventral tegmentum > or = colliculi = hypothalamus = hippocampus > brain stem > or = cerebellum > or = dorsal horn of the spinal cord > cerebral cortex > or = ventral horn of the spinal cord > olfactory tubercle. This distribution was confirmed by in situ hybridization, which further revealed that the 5-HT1B mRNA was present in dorsal root ganglia, the layer IV of the cerebral cortex, the Purkinje cell layer of the cerebellum, the pyramidal neurons in the CA1 area of the hippocampus, and the dorsal and median raphe nuclei. In situ hybridization was also performed in nomifensine (10 mg/kg/i.p.)-pretreated rats whose serotoninergic neurons were extensively and selectively lesioned by microinjection of 5,7-dihydroxytryptamine (8 micrograms/1 microliter) directly into the anteroventral vicinity of anterior raphe nuclei 3 weeks before sacrifice. In lesioned rats, 5-HT1B mRNA was present in the same areas and at the same levels as in control rats, except in the dorsal and median raphe nuclei, where a marked decrease (-75%) in its local concentration was observed. These data provide the first demonstration of the synthesis of 5-HT1B receptor within serotoninergic neurons, as expected of their presynaptic autoreceptor function at the level of serotoninergic terminals.