Mammalian circadian organization is believed to derive primarily from circadian oscillators within the hypothalamic suprachiasmatic nuclei (SCN). The SCN drives circadian rhythms of a wide array of functions (e.g., locomotion, body temperature, and several endocrine processes, including the circadian secretion of the pineal hormone melatonin). In contrast to the situation in several species of reptiles and birds, there is an extensive literature reporting little or no effect of pinealectomy on mammalian circadian rhythms. However, recent research has indicated that the SCN and circadian systems of several mammalian species are highly sensitive to exogenous melatonin, raising the possibility that endogenous pineal hormone may provide feedback in the control of overt circadian rhythms. To determine the role of the pineal gland in rat circadian rhythms, the effects of pinealectomy on locomotor rhythms in constant light (LL) and constant darkness (DD) were studied. The results indicated that the circadian rhythms of pinealectomized rats but not sham-operated controls dissociated into multiple ultradian components in LL and recoupled into circadian patterns only after 12-21 days in DD. The data suggest that pineal feedback may modulate sensitivity to light and/or provide coupling among multiple circadian oscillators within the SCN.