Using perifused explants of the rat suprachiasmatic nucleus (SCN), the effects of tetrodotoxin (TTX) on vasopressin (VP) release and its circadian profile were studied at various times throughout the circadian cycle. VP release from SCN explants was consistently attenuated during TTX treatment, with the amplitude of this effect depending on the time of administration. In addition, the effect of TTX on the circadian pattern of VP release was also time-dependent, such that treatment during the late subjective day was followed by a disruption of circadian rhythmicity in which peptide output remained at basal levels without notable variation whereas treatment at all other times caused no measurable perturbation in the circadian VP rhythm in succeeding cycles. In SCN explants experiencing this TTX-induced arrest of circadian VP output, subsequent exposure to KCl induced acute increases in VP release, suggesting that VP neurons retain the capacity to actively release peptide in spite of this effect of TTX. These results indicate that the interruption of electrical impulses at a critical phase may compromise the circadian function or output of the pacemaker in the SCN. In addition, the present observations provide further evidence that the overt expression of circadian rhythmicity is dependent on sodium-generated action potentials and that disruption of these electrical signals does not alter the precision of the SCN pacemaker, at least in instances where the phase of the VP rhythm can be determined after treatment.