The periaqueductal gray plays an important role in the descending modulation of nociception. While the importance of endogenous opioids to periaqueductal gray circuits that modulate nociception is supported by many studies, the ultrastructural relationships between enkephalin-immunoreactive axon terminals and the surrounding periaqueductal gray neuropil have not been quantitated in the rat. Further, the possible interaction between enkephalin-immunoreactive axon terminals and periaqueductal gray neurons that project to the rostroventral medulla has not been described. The present study utilized electron microscopic immunocytochemistry to quantitate the normal neuronal associations of enkephalinimmunoreactive terminals in the caudal periaqueductal gray of the rat. A primary focus of this analysis was to ascertain whether any interaction exists between enkephalin-immunoreactive axon terminals and periaqueductal gray neurons that were retrogradely-labeled from the nucleus raphe magnus and adjacent medullary reticular nuclei. We examined the ventrolateral periaqueductal gray and the ventral periaqueductal gray immediately subjacent to the aqueduct and found that both the average terminal diameters and the volume fractions of enkephalin-immunoreactive terminals were very similar. In these two regions, most terminals were observed to be in close apposition to either two or three dendrites that were neither retrogradely-labeled nor enkephalin-immunoreactive, although axonal and perikaryal associations were also observed. In the ventrolateral periaqueductal gray, 22% of all enkephalin-immunoreactive terminals were adjacent to periaqueductal gray-nucleus raphe magnus and periaqueductal gray-reticular nucleus projection neurons. In the periaqueductal gray subjacent to the aqueduct, 32% of all enkephalin-immunoreactive terminals were adjacent to periaqueductal gray-nucleus raphe magnus and periaqueductal gray-reticular nucleus projection neurons. Symmetrical synapses with these retrogradelylabeled neurons were formed by 5.5% of enkephalin-immunoreactive terminals in the ventrolateral periaqueductal gray, and by 4.3% of enkephalin-immunoreactive terminals located subjacent to the aqueduct. We also noted that enkephalin-immunoreactive terminals formed symmetrical synapses with non-retrogradely-labeled, enkephalin-immunoreactive dendrites in the periaqueductal gray. Direct opioid input onto putative excitatory periaqueductal gray output neurons that are hypothesized to modulate nociception was an unexpected finding. While presynaptic mechanisms for enkephalin action on periaqueductal gray output neurons may potentially exist, the low frequency of terminal-terminal interactions compared with the high frequency of terminal-dendrite interactions suggested a significant role of enkephalins via either disinhibitory or direct inhibitory mechanisms. These results are discussed in light of current theories about nociception-modulating circuits within the periaqueductal gray.