Considerable evidence has demonstrated an interrelationship between the enkephalinergic and dopaminergic systems in both the mammalian and invertebrate nervous systems. We have described recently the presence of two classes of high affinity opiate binding sites in the nervous tissue of the marine mollusc Mytilus edulis. In order to examine the physiological role of these high affinity opiate sites, M. edulis pedal ganglia (Pg) were treated with the selective neurotoxin 6-hydroxydopamine (1 micrograms/animal, applied topically to the intact Pg); animals were sacrificed 5 days after treatment. The dopamine content of the Pg from lesioned animals was reduced to 33% relative to that of Pg from control animals. Neither serotonin nor norepinephrine levels were reduced. Fluorescent micrographs of formaldehyde-treated Pg from both lesioned and control animals revealed that the neurotoxic substance accumulates in the synaptically rich neuropil and not in the cortex of the Pg. Thus, the partial reduction in dopamine levels may reflect nearly total loss of dopamine in terminals with essentially no change in the nerve cell bodies. High affinity binding of the potent opioid peptide 125I-labeled FK 33-824 (2 nM) was reduced by 81% and low affinity binding (10 nM peptide) by 43% in Pg from lesioned animals relative to that in control tissue. In addition, D-Ala2-Met5-enkephalin, beta-endorphin and etorphine failed to change dopamine levels in lesioned animals. Together, these results suggest that the high affinity opiate binding sites that mediate alteration in dopamine levels are on dopaminergic presynaptic terminals.