The suprachiasmatic nucleus (SCN) is believed to play a major role in the generation and control of circadian rhythms in mammals. In order to obtain further evidence concerning this, single and multiple neuronal discharges were continuously recorded over a period of several days in neonatal rat SCN explants. These organotypic explants, which had been cultured for several weeks in a chemically defined medium, showed alternating high and low levels of spontaneous neuronal discharges with a periodicity around 24 h. Such explants can serve as a useful model to study the neuronal mechanisms underlying the generation of mammalian circadian rhythms.