Fluoride-resistant acid phosphatase (FRAP)-reactive terminals making contact with interneuronal soma are found in the substantia gelatinosa of the mouse spinal dorsal horn. About one half of the interneuronal somata receive FRAP-positive boutons. By electron microscopy, these FRAP-positive terminals appear small, dark, slender, roundish, cap-like, ellipsoid or sinuous and electron-dense, scalloped (fan-like) contours with clear spherical synaptic vesicles of variable size, some large dense-core vesicles and mitochondria. All these features are very similar to those of capsaicin-sensitive terminals. Thus they are considered to be nociceptive primary afferent endings. Therefore, some of the FRAP-positive terminals are suggested to have a modulatory role in the nociceptive circuit in the substantia gelatinosa.