Summary: FRAP (fluoride-resistant acid phosphatase) -reactivity in the substantia gelatinosa of the mouse spinal trigeminal nucleus caudalis (STNC) was examined by light and electron microscopy. Degenerated figures of terminals caused by capsaicin were compared with the FRAP-positive terminals. Scalloped (fan-like) or indented, sinuous, slender, and cap-like figures with closely packed agranular synaptic vesicles of various sizes were common to both FRAPpositive and capsaicin-sensitive terminals. These terminals had glomerular or nonglomerular endings. Sometimes FRAP-positive and capsaicin-sensitive glomerular terminals made presynapses with surrounding dendrites. Frequently, both nonglomerular terminals were in direct contact with the neuronal soma. The terminal features of FRAP-positive and capsaicin-sensitive ones in the mouse STNC are the same as those seen in the superficial dorsal horn of the spinal cord. These findings suggest that some of the FRAP-positive terminals are capsaicin-sensitive, thereby indicating their nociceptive primary afferent.FRAP (fluoride-resistant acid phosphatase), a nonlysozomal enzyme, is localized to small dorsal root ganglion (DRG) neurons. After dorsal root ligation (Knyihar, 1971), dorsal rhizotomy, blocking the axonal flow by microtubule polymerizing agents such as colchicin and vinblastine (Knyihar-Csillik and Csillik, 1981) and capsaicin treatment (Nagy et aL, 1981), FRAP reactivity disappeared in lamina II of the spinal dorsal horn and small DRG neurons. Accordingly, FRAP is believed to be synthesized in small primary afferent neurons and conveyed to the central (Knyihar, 1971) and peripheral (Devor and Claman, 1980) terminals by axonal flow. Thirty to 50% of the DRG cells projecting to the splanchnic or intercostal nerve are FRAPpositive (Dalsgaard et al., 1984). Moreover, numerous unmyelinated axons innervating the corneal epithelium exhibit acid phosphtase activity (Knyihar-Csillik and Csillik, 1981). Therefore, FRAP has been currently used for identification of the primary afferent neurons. The fine structures of FRAP-positive central endings of the primary afferent were extensively documented by KnyiharCsillik and Csillik (1981). The FRAP-positive terminals in the rat substantia gelatinosa of the spinal dorsal horn and caudal spinal trigeminal nucleus are dark sinusoid axons (DSA) or scalloped (fanlike) terminals with packed regular (clear) synaptic vesicles and few or numerous large dense core vesicles. Ribeiro-da-Silva et al. also examined FRAP-positive terminals in the substantia gelatinosa of the rat spinal dorsal horn (1986) and spinal trigeminal nucleus caudalis (1989). They showed that FRAP-positive terminals have electron-dense sinuous or scalloped contours with closely packed regular synaptic vesicles, few large dense core vesicles and mitochondria. Fine structures of the FRAP-positive terminals are the same as the central terminals (CI type), described by Ribeiro-daSilva and Coimbra in 1982, situated centrally forming synaptic glomeruli which are capsaicin...