The suprachiasmatic nucleus has been identified tentatively as a circadian pacemaker. To examine the functional role of peptides found within suprachiasmatic neurons, avian pancreatic polypeptide and vasopressin were microinjected into the suprachiasmatic region. Avian pancreatic polypeptide, but not vasopressin, shifted the phase of the wheelrunning rhythm as a function of the time of its injection within the circadian cycle. Avian pancreatic polypeptide or a similar peptide may be one component of the neurochemical processes underlying entrainment to the light-dark cycle.