The eye of the marine mollusk Aplysia californica contains a photo-entrainable circadian pacemaker that drives an overt circadian rhythm of spontaneous compound action potentials in the optic nerve. Both light and serotonin are known to influence the phase of this ocular rhythm. The current study evaluated the effect of FMRFamide on both light and serotonin induced phase shifts of this rhythm. The application of FMRFamide was found to block serotonin induced phase shifts but, by itself, FMRFamide did not cause significant phase shifts. Furthermore, the effects of FMRFamide on lightinduced phase shifts appeared to be phase dependent (i.e., the application of FMRFamide inhibited lightinduced phase delays but actually enhanced the magnitude of phase advances). As in Aplysia, the eye of Bulla gouldiana also contains a circadian pacemaker. In Bulla, FMRFamide prevented light-induced phase advances and delays. Although FMRFamide alone generated phase dependent phase shifts, it did not cause phase shifts at the phases where it blocked the effects of light. These data demonstrate that FMRFamide can have pronounced modulatory effects on phase shifting inputs to the ocular pacemakers of both Aplysia and Bulla.