The primary mammalian circadian clock is located within the suprachiasmatic nuclei (SCN), but the cellular organization of the clock is not yet known. We investigated the potential role of glial cells in the clock mechanism by determining whether disrupting glial activity affects the in vitro circadian rhythm of neuronal activity and the in vivo circadian activity rhythm in rats. We used two agents (octanol and halothane) that block gap junctions, and one (fluorocitrate) that inhibits glial metabolism. All three agents disrupted the circadian pattern of neuronal activity. Octanol flattened the rhythm at the highest concentration (200 microM) and induced a small phase delay at a lower concentration (66 microM). Halothane and fluorocitrate induced ultradian rhythmicity. Fluorocitrate injected into the SCN of an intact rat induced arrhythmicity for about 1 week, after which the rhythm reappeared with a 1.6 h delay. These results suggest that glia play an important role in the SCN circadian clock.