Using a perifusion culture technique, the pattern of vasopressin release from the suprachiasmatic nucleus was studied in vitro under basal and stimulated conditions. Individual rat suprachiasmatic explants released vasopressin in a rhythmic fashion and these rhythms persisted with a circadian period for up to 4 cycles. One-hour exposures to KCl on days 2 and 4 in culture induced acute increases in vasopressin output and altered the circadian pattern of release from perifused suprachiasmatic explants. These results provide evidence that the release of vasopressin in an acute as well as a circadian fashion from isolated suprachiasmatic neurons is indicative of active release processes. Furthermore, it is anticipated that this method of studying circadian rhythms generated by suprachiasmatic explants may provide a basis for elucidating the neural organization of mammalian circadian pacemakers.