Neuronal activity of a single neuron was monitored continuously for more than 5 days by means of a multi-electrode dish in dispersed cell culture of the rat suprachiasmatic nucleus (SCN). Sixty-seven out of 88 neurons showed a robust circadian rhythm in firing rate. The mean circadian period was 24.2 h, which was almost identical to that of the locomotor activity rhythm in 114 weanling rats blinded on the day of birth. However, the circadian period in individual SCN neurons was scattered from 20.0 to 28.3 h (SD, 1.4 h), while the period of activity rhythm clustered from 24.0 to 24.8 h (SD, 0.2 h). It is concluded that a large number of SCN neurons contain the circadian oscillator, the period of which is more variable than the circadian period of the SCN as a whole. It is suggested that the circadian rhythms in individual SCN neurons are capable of synchronizing to each other and are integrated to constitute a multiple oscillator system(s) within the SCN.