Previous studies showed that spinothalamic tract (STT) neurons in the dorsal horn head have a distinct laminar organization that corresponds to the cytoarchitectonic divisions. However, groupings of STT neurons in the deeper part of the spinal gray matter have not yet been established, since, in these regions, the distribution of STT neurons with similar morphology does not follow the cytoarchitectonic subdivisions. To classify STT neurons in the deep part of the rat spinal gray matter, not only the distribution but also the size and shape of STT neurons were investigated in the present study by the method of retrograde axonal transport of wheat germ agglutinin-horseradish peroxidase. In addition to the confirmation of the previously described groups of STT neurons in the dorsal horn head, this study showed novel groupings of STT neurons in the deeper part of the spinal gray matter. First, the interomedio-ventral group of STT neurons, which spreads over laminae VII through IX, were classified into four subgroups in the cervical cord according to their side of projection and medio-lateral location: ipsilateral-medial, ipsilateral-lateral, contralateral-medial and contralateral-lateral. These subgroups were different not only in distribution of neurons but also in mean size of their cell bodies. On the ipsilateral side, neurons in the lateral subgroup were smaller than those in the medial subgroup, while on the contralateral side, the lateral subgroup had larger neurons than the medial subgroup. Second, STT neurons in laminae IV-VI were classified into the dorsal horn neck and base groups based on the orientation of their somata and their rostrocaudal distribution. Neurons in the dorsal horn neck group were more densely distributed and more horizontally aligned than those in the dorsal horn base group. The dorsal horn base group contained more STT neurons projecting to the medial thalamus than the dorsal horn neck group.