In mammals, circadian rhythms are driven by a pacemaker located in the suprachiasmatic nuclei (SCN) of the hypothalamus. The pacemaker is composed of an ensemble of multiple, single-cell oscillators in the SCN. We measured arginine–vasopressin (AVP) release in organotypic SCN slices. The SCN slice culture showed circadian oscillation of AVP release with a period length (± SEM) of 23.84 ± 00.03 h. This period is very similar to the one we previously reported in dispersed SCN cultures and is also close to that of behavioural rhythms. When the ventral part was removed by a surgical cut across the slice in the horizontal plane, however, the period became shorter (23.22 ± 00.08 h). On the other hand, the removal of the dorsal part did not affect period length. These results suggest that the oscillators in ventral and dorsal cells contribute differently to period length and that the dorsal oscillators are entrained by the ventral ones to form a single integrated oscillator.