The eye of the marine mollusk Aplysia californica contains a photo-entrainable circadian pacemaker that drives an overt rhythm of spontaneous compound action potentials. The current study evaluated the influence of serotonin on light-induced phase shifts of this ocular rhythm. The application of serotonin in combination with light was found to have profound and interactive effects on the magnitude of the resulting phase shifts. Further, the phase shifts that resulted from the interaction between light and serotonin appeared to be phase dependent, i.e., the application of serotonin inhibited the phase shifting effects of light during one part of the circadian cycle but enhanced them during another. Finally, the results show that the interaction between light and serotonin is dependent upon the sequence in which these two treatments are paired. These data, coupled with previous findings, suggest that serotonin may act to modulate light's phase shifting effects on the ocular pacemaker in Aplysia.