Intraventricular injections of the neurotoxin, 5,7-dihydroxytryptamine (DHT), were used to lesion hamster forebrain serotonin systems. The entrained circadian wheelrunning rhythm was studied for up to 20 weeks post-lesion as was the extent of reinnervation of nuclei regulating circadian rhythmicity. Reinnervation of the suprachiasmatic nucleus and intergeniculate leaflet by serotonergic fibers begins by 8 weeks and progresses to substantial, but not complete, levels by week 20. Four measures of the nocturnal activity phase of the circadian rhythm were rapidly modified by the lesions, but in contrast to the morphology, persisted unchanged during the entire 20 week test period. The circadian rhythm system of hamsters may be fundamentally different from other behavioral or neuroendocrine systems studied in rats with respect to its inability to recover from damage to its serotonergic innervation. Alternatively, the failure to demonstrate functional recovery may reflect a species difference or insufficient recovery time.