HRP solution was injected into the right subnucleus caudalis of the trigeminal nerve in the cat, and the labeled cells in the nucleus raphe dorsalis of six cases were examined and analyzed. The labeled cells appeared more in the middle portion and its rostral adjacent parts of the nucleus raphe dorsalis, decreased in number rostrally and caudally, and were entirely absont in the caudal end of the nucleus. In the ventral part and the ventral areas of the lateral parts of the nucleus labeled cells were more than in the other areas of the nucleus, no difference was found between the two sides. In the dorsal part and the dorsal regions of the lateral parts of the nucleus only a few labeled cells were seen. The labeled cells varied in forms, but in the ventral part and the ventral regions of the lateral parts they were mainly fusiform in shape. In addition, in nucleus raphe magnus, nucleus raphe pallidus, and nucleus raphe obscurus, small numbers of neurones were labeled as well.As the nucleus raphe magnus takes part in the inhibition of the response to noxious stimuli in the spinal dorsal horn, and as both the nucleus raphe magnus and nucleus raphe dorsalis belong to the more developed raphe nuclei and contain more 5-HT neurones, it seems that there might be somatotopic relations between the nucleus raphe dorsalis which projects mainly to subnucleus caudalis and the nucleus raphe magnus which projects mainly to the spinal dorsal horn.