The mammalian circadian pacemaker in the suprachiasmatic nuclei (SCN) receives a dense input from histamine-containing neurons in the posterior basal hypothalamus. We applied histamine to SCN tissue in vitro and measured the subsequent rhythm in firing rate. Histamine caused a phase delay in the early subjective night and a phase advance in the late subjective night. The similarity of histamine- and photic-induced phase shifts indicates that histamine may play a role in the modulation of circadian clock photic input.