To explore the multioscillator nature of the neurohumoral circuitry controlling the expression of circadian rhythmicity, rats' wheel running circadian activity rhythms were compared following sagittal knife cuts in the region of the suprachiasmatic nucleus (SSCN), following pinealectomy (PX) and following the combination of SSCN and PX. 25% of animals with knife cuts that passed through one SCN had disturbed running activity under constant illumination; rhythmic disturbances were seen neither in animals with sham knife cuts nor in rats with knife cuts on the midline or lateral to the SCN. Animals with both SSCN and PX were twice as likely to show severe rhythmic disruptions under free-running conditions as rats with SSCN and sham PX. Rats with PX and sham SSCN did not display disrupted activity rhythms. When animals with PX alone or SSCN alone were first observed under free-running conditions and then subjected to a second surgical procedure so that all animals underwent both PX and SSCN, all PX and most SSCN animals demonstrated coherent activity rhythms after the first operation, but 35% showed disruptions in circadian activity patterns only following the second surgery. The activity rhythms of rats with knife cuts placed either on the midline or lateral to the SCN did not deteriorate when combined with PX. Rats with coherent rhythms following knife cuts damaging one SCN had rhythm disruptions after the addition of PX. The effects of pinealectomy may indicate that the pineal gland plays a role in maintaining the coupling relationships in the multioscillator system controlling circadian activity rhythms. The results of this study also suggest that neither the direct commissural connection of the SCNs nor the humoral output of the pineal gland is indispensable for the expression of coherent circadian activity rhythms in the rat.