The necessity of a circadian pacemaker, the suprachiasmatic nuclei (SCN), for survival was evaluated in a population of approximately 65 wild eastern chipmunks, Tamias striatus. The research involved over 3000 h of fieldwork between May 1995 and October 1997 on a study siteat Mountain Lake Biological Station, Virginia. The 28 chipmunks randomly designated as project animals included 10 SCN-lesioned chipmunks, 5 surgical controls (shamlesioned), and 13 intact controls. Visual observation, live trapping, and radio telemetric tracking were used to assess 6aspects of survival and reproduction. Upon release after surgery, every animal returned to its den site and was able to maintain its home territory. In warm months from May through October, all chipmunks were active above ground with a strictly day-active pattern. During the remainingcold months, they were normothermic but relatively torpid in their underground densfor extendedperiods of time. Short-term mortality for the initial 3 months included only a single intact control chipmunk; loss for the extended period from August 1995 to October 1996 was 40% for the SCN-lesioned animals, 0% for surgical controls, and 15.4% for the intact controls.Survival differences were not significant between surgical control and intact control groups but were significant ([.alpha] = .10) between SCN-lesioned and pooled control groups. Annual bodyweight patterns were similar for both groups. Most individuals in both SCN-lesioned and controlgroups were reproductively active in the spring and fall breeding periods. Lack of major differences may be attributable to the exceptionally favorable conditions for survival such as a veryabundant fallacorn crop, medium population size, and absence of heavy predation.