We have developed a dissociated suprachiasmatic nucleus (SCN) cell culture system in order to begin a cellular analysis of the mammalian circadian oscillator. Monolayer-cultured neurons prepared from the SCN regions of 1-day-old rats were maintained in serum-free culture medium for 3-4 weeks and then perfused. In 12 out of 40 wells, a circadian oscillation of vasopressin release persisted for at least 4-5 cycles until the end of the perfusion period with no damping of the amplitude. This finding suggests that cultured SCN neurons retain the capacity for circadian oscillation for a long time, and should provide a useful model for the analysis of a mammalian circadian system at the cellular level.