a quantitative light microscopic analysis and ultrastructural description of cholecystokininlike immunoreactivity in the spinal trigeminal nucleus of the rat the spinal trigeminal nucleus is involved in orofacial sensory transmission cholecystokinin octapeptide has been identified in axons in this nucleus and appears to play a role in the transmission of orofacial sensation from the trigeminal ganglia to the spinal trigeminal nucleus although cholecystokinin has been reported in axonal processes within the spinal trigeminal nucleus at the light microscopic level nothing is known about the synaptic relationships of these cholecystokinin axons the goals of this study were to quantitatively determine the volume fraction of cholecystokininlike immunoreactive cell bodies and fibers in the three subnuclei of the spinal trigeminal nucleus to provide the first ultrastructural description of cholecystokininlike immunoreactive processes within these subnuclei and to analyse the synaptic relationships of cholecystokininlike immunoreactive processes within the spinal trigeminal nucleus neuropil cholecystokininlike immunoreactivity was localized by the peroxidaseantiperoxidase method or the peroxidase labeled avidinbiotin technique and quantified at the light microscopic level by point counting immunoreactive fibers were present in all three subnuclei but the greatest volume fraction of immunoreactive axons was obtained in laminae i and ii of the nucleus caudalis no immunoreactive cell bodies were evident in any of the subnuclei the majority of immunoreactive profiles in all three subnuclei were identified ultrastructurally as axon terminals that contained both small and medium sized agranular vesicles and infrequently large dense core vesicles these immunoreactive terminals were usually found in close contact with nonimmunoreactive dendrites with which they were observed to form asymmetric synapses immunoreactive terminals were occasionally observed to contact the cell bodies of large nonimmunoreactive neurons on the border of laminae i and ii in the nucleus caudalis these results indicate that cholecystokininlike immunoreactive processes are present throughout the spinal trigeminal nucleus and in nucleus caudalis show a distribution similar to that reported for the spinal cord dorsal horn immunoreactive axons make synaptic contact with both the dendrites and perikarya of spinal trigeminal nucleus neurons no axoaxonic synapses were observed these findings suggest that cholecystokinin plays an important role in spinal trigeminal nucleus function the possible colocalization of cholecystokinin and substance p in the spinal trigeminal nucleus and the possible role of cholecystokinin in attenuating the action of opioids in the spinal trigeminal nucleus are also discussed