The effect of neuropeptide Y (NPY) on the spontaneous discharge of neurons within the hamster (Mesocricetus auratus) suprachiasmatic nucleus (SCN) (N = 83) was determined using the hypothalamic slice preparation. The discharge of neurons within the ventral SCN recorded during the day was either excited (6/42), immediately inhibited (17/42), or transiently excited and then inhibited (10/42) by NPY 10(-7). During the night, NPY produced excitatory effects in 2/23 neurons, inhibition in 7/23 and excitation followed by inhibition in 4/23. A higher percentage of neurons was found to be unresponsive to NPY during the night than during the day. This difference approached but did not reach statistical significance. In the 18 neurons recorded within the dorsal SCN, NPY had little effect on spontaneous discharge during the day or night. These data indicate that bath application of NPY predominately inhibits the spontaneous discharge of SCN neurons recorded in the hypothalamic slice preparation.