The retinohypothalamic tract (RHT) is a direct pathway from the retina to the suprachiasmatic nucleus (SCN). Electrical stimulation of the optic nerve or optic chiasm activates the RHT and produces shifts in phase of a circadian rhythm in SCN neuron activity in rat hypothalamic slices in vitro. The phase response curve (PRC) for this effect is very similar to that obtained from administration of light pulses to intact animals maintained in constant darkness. The effect of optic chiasm stimulation is blocked by tetrodotoxin. In addition to the RHT, there is a second entraining pathway, the geniculohypothalamic tract, which arises from neuropeptide Y (NPY)-containing neurons of the intergeniculate leaflet of the lateral geniculate complex. In contrast to optic chiasm stimulation. NPY produces phase shifts in the rhythms of SCN neuron firing rate in vitro with a PRC that similar to that for NPY infusion into the SCN in intact animals as well as that produced by a series of treatments that induce locomotor activity. These results indicate that phase shifts of the circadian rhythm of SCN neuron activity may be produced by activation of two different entraining pathways and that the physiological actions of these pathways on pacemaker function are markedly different.