Enkephalins are opioid peptides which elicit an analgesic effect when injected into the periaqueductal gray matter and in particular into its caudal part. In this study we wanted to investigate the fine structure of the neurons and of the synaptic circuits involved in the enkephalin action in rats using immunocytochemical pre-embedding methods. The labelled neurons show a small-medium sized soma, with a large nucleus, an absence of an extensive granular endoplasmic reticulum and few synaptic contacts. The synapses on enkephalin dendrites are often multiple, unlabelled and show both symmetrical and asymmetrical junctions. Enkephalin axon terminals make synaptic contacts on dendrites, usually unlabelled; contacts between enkephalin elements were occasionally seen. The enkephalin synapses show a symmetrical type of junction. These data give morphological support to the physiological findings, in particular to those suggesting an inhibitory effect of enkephalin on periaqueductal gray matter cells through a post-synaptic process. The findings are discussed with respect to pain modulation taking into consideration the enkephalin synaptic complex in intrinsic circuits and the possible ending of enkephalin axons on the projective neurons of the periaqueductal gray matter.