Circadian rhythms are modeled as reliable and self-sustained oscillations generated by single cells. The mammalian suprachiasmatic nucleus (SCN) keeps near 24-h time in vivo and in vitro, but the identity of the individual cellular pacemakers is unknown. We tested the hypothesis that circadian cycling is intrinsic to a unique class of SCN neurons by measuring firing rate or Period2 gene expression in single neurons. We found that fully isolated SCN neurons can sustain circadian cycling for at least 1 week. Plating SCN neurons at <100 cells/mm 2 eliminated synaptic inputs and revealed circadian neurons that contained arginine vasopressin (AVP) or vasoactive intestinal polypeptide (VIP) or neither. Surprisingly, arrhythmic neurons (nearly 80% of recorded neurons) also expressed these neuropeptides. Furthermore, neurons were observed to lose or gain circadian rhythmicity in these dispersed cell cultures, both spontaneously and in response to forskolin stimulation. In SCN explants treated with tetrodotoxin to block spike-dependent signaling, neurons gained or lost circadian cycling over many days. The rate of PERIOD2 protein accumulation on the previous cycle reliably predicted the spontaneous onset of arrhythmicity. We conclude that individual SCN neurons can generate circadian oscillations; however, there is no evidence for a specialized or anatomically localized class of cell-autonomous pacemakers. Instead, these results indicate that AVP, VIP, and other SCN neurons are intrinsic but unstable circadian oscillators that rely on network interactions to stabilize their otherwise noisy cycling.luciferase ͉ pacemaker ͉ Period gene ͉ suprachiasmatic nucleus ͉ vasoactive intestinal polypeptide C ircadian pacemakers are schematized as intracellular transcription-translation negative feedback loops (1). In mammals, transcription factors including CLOCK and BMAL1 promote the expression of clock genes, including Period 1 (Per1) and 2 (Per2). The protein products of these genes return to the nucleus after a delay of many hours to repress their own transcription. Genetic deletion of these repressors abolishes circadian rhythms in behavior and physiology (2). The strongest evidence for cell-autonomous, circadian rhythm generation in mammals comes from transcriptional rhythms measured from primary and immortalized fibroblasts (3, 4).The mammalian suprachiasmatic nucleus (SCN) of the anterior hypothalamus coordinates daily rhythms including sleep-wake and hormone release (5). Multielectrode array (MEA) recordings of neuronal firing and luciferase-based reporters of Per1 and Per2 expression showed dissociated SCN neurons in the same culture with different circadian periods (6, 7). Furthermore, Na ϩ -dependent action potentials, vasoactive intestinal polypeptide (VIP), and its receptor, VPAC2, are required for cellular synchrony and maintaining daily oscillations across the SCN (8, 9). Taken together, these results suggest that single SCN neurons are competent circadian oscillators. However, which, if any, SCN neurons are capable ...