Daily exercise promotes physical health as well as improvements in mental and neural functions. Studies in intact wild-type (WT) rodents have revealed that the brain's suprachiasmatic nuclei (SCN), site of the main circadian pacemaker, are also responsive to scheduled wheel running. It is unclear, however, if and how animals with a dysfunctional circadian pacemaker respond to exercise. Here, we tested whether scheduled voluntary exercise (SVE) in a running wheel for 6 hours per day could promote neural and behavioral rhythmicity in animals whose circadian competence is compromised through genetically targeted loss of vasoactive intestinal polypeptide (VIP(-/-) mice) or its VPAC(2) receptor (Vipr2(-/-) mice). We report that in constant dark (DD), rhythmic VIP(-/-) and Vipr2(-/-) mice show weak free-running rhythms with a period of <23 hours and all wild-type mice are strongly rhythmic with approximately 23.5-hour periodicity. VIP(-/-) and Vipr2(-/-) mice rapidly (<7 days) synchronize to daily SVE, while WT mice take much longer (>35 days). Following 21 to 50 days of SVE, WT mice show small changes in their rhythms, and most Vipr2(-/-) mice now sustain robust near 24-hour behavioral rhythms, whereas very few VIP(-/-) mice do. This study demonstrates that scheduled daily exercise can markedly improve circadian rhythms in behavioral activity and raises the possibility that this noninvasive approach may be useful as an intervention in clinical etiologies in which there are dysfunctions of circadian time keeping.