Like several other opisthobranch molluscs, the marine snail Bulla gouldiana possesses two circadian pacemakers, one in each eye. The two ocular pacemakers are mutually coupled such that: the circadian rhythms of spontaneous electrical activity recorded from the optic nerve are normally synchronous and; if experimentally desynchronized the rhythms will return to the synchronized state. This coupling of the pacemakers is mediated by efferent fibers in the optic nerve, terminating in neuropil adjacent to the basal retinal neurons (BRNs), the putative circadian pacemaker cells. Attempts to identify neurotransmitters in efferent terminals that may be involved in the coupling process have failed. In the present study we demonstrate axons in the optic nerve and axon terminals adjacent to the BRNs that exhibit FMRF-amide- (molluscan cardioexcitatory peptide) and NPY-like (neuropeptide-Y) immunoreactivity. The pattern of immunoreactivity to both antisera is identical. Blocking studies indicate that both antisera are recognizing the same site, most likely the arginine-phenylalanine-amide terminus of FMRF, or an FMRF-like molecule. We conclude that FMRF is a candidate for the chemical mediator involved in the interaction between the two ocular pacemakers in Bulla gouldiana.