The suprachiasmatic nucleus (SCN) of the hypothalamus constitutes a major pacemaker for circadian rhythms in mammals. Rod outer segment (ROS) disk shedding follows a circadian rhythm that persists after pre-chiasmatic optic nerve section (ONS) (Teirstein et al., Invest. Ophthalmol. Vis. Sci., 19 (1980) 1268-1273), suggesting local oscillatory control. However, entrainment and phase-shifting to light-dark (LD) cycles are disrupted, suggesting that central efferent signals are involved in phase control. In the present experiment, lesions of the SCN were administered to 45 rats, resulting in complete behavioral arrhythmicity in 24 rats, dampened rhythmicity in 11 rats and robust rhythms in 10 rats. The extent of lesions was verified by histological and immunocytochemical methods. After screening for presence or absence of activity rhythms, rats were assigned to separate groups for ROS disk shedding analysis at four phases relative to the (LD) 12:12 cycle: 1.5 h after light onset, 1.5 h after dark onset, and at corresponding phases immediately upon release into 24 h of constant darkness. The disk shedding pattern, with relatively high phagosome numbers during the early light phase and low numbers at nighttime, was similar for behaviorally arrhythmic, dampened and rhythmic groups. These data indicate an independence of the disk shedding rhythm from possible SCN humoral or neuronal efferent influences.