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Laboratory experiments, based on knowledge of the natural environment, have been conducted to detemine the role of various factors in the entrance into and emergence from hibernation. Woodchucks, captured in the wild were kept in rooms at several temperatures or in coolers at 6C. They were fed or deprived according to the experimental program. Experiments showed that woodchucks deprived of food and kept at 6°C became torpid but woodchucks fed at 6°C did not. Some woodchucks deprived of food at 20°C became lethargic in winter. Minor disturbances in the laboratory do not arouse woodchucks. Woodchucks that eat when aroused do not become torpid again unless again deprived. Thin woodchucks become torpid as readily as fat ones. The latency of torpidity varies with season, being short (a week) in December and March and long (a month) in summer. The woodchuck has bouts of torpor that are short (4 to 5 days) at first but become longer (10 to 12 days). The arousal bouts last 2 to 3 days. Hibernation in nature lasts 3 to 4 months although presumably the woodchuck is toprid for only 2 months. In the laboratory woodchucks may remain in hibernation (alternating bouts of torpor and arousal) up to 8 months. The stimulus for final arousal was not determined. Energence from the burrow at Chambersburg, Pennsylvania occurred between January 29 and 8 February in 8 years. The date had no relation to local weather. Woodchucks exposed to very low temperatures (–20oC) aroused promptly.