The suprachiasmatic nucleus (SCN) of the hypothalamus is involved in the generation and entrainment of circadian rhythms. The results of a series of experiments in long-term cultured organotypic SCN slices suggest that (1) some but not all SCN neurons display circadian rhythmicity in their extracellular discharges. To the extent they could be studied, these neurons did not usually show synchronized high/low levels of activity; (2) simultaneous daily depolarization of these neurons (K+ pulses) to some extent influenced the distribution of the firing rate of SCN neurons around the time of expected daily pulses; (3) extracellular Ca++ and synaptic input is required for the pacemaker activity of the SCN. We conclude that the mammalian biological clock is a heterogeneous neuronal system in which the circadian pacemaker rhythm is generated and entrained via complex interactions among SCN neurons.