Seasonal changes in body weight, carcass composition, food habits, and general behavior were determined in Spermophilus beldingi beldingi, a hibernatory ground squirrel that lives at high altitude. Effects of photoperiod and constant or cycling ambient temperature on body weight regulation were tested in the laboratory. There was about a two-fold annual variation in body weight. Most of this variation was due to fluctuations in fat stores. Basic (fat-free, dry) weight was quite constant throughout the season. Carcass water content varied in that squirrels became slightly dehydrated during hibernation and total body water was inversely related to total body lipid. Prehibernatory fattening occurred over a period of about seven weeks in the population of adults at a given location and involved about a 15-fold increase in total body lipid. In individuals, the response took only about five weeks in males and three weeks in females. All individuals hibernated for about nine months. Body weights decreased during hibernation by as little as 33 percent in yearling females and as much as 43 percent in adult males. Even so, all squirrels emerged with 20 to 25 percent of total prehibernatory lipid stores intact. The schedule of prehibernatory fattening varied by as much as four weeks between individual years. This variation seemed to be related directly to snowpack. The more snow, the later fattening occurred. Following the first weeks of snow disappearance, food was plentiful. Many squirrels fattened and entered hibernation before or at the peak of vegetation abundance. Green grass was the main food item but arthropods were taken throughout the season and large quantities of seeds were eaten when they became available. A greater percentage of time above ground was spent feeding during the fattening phase than earlier in the season. Neither photoperiod nor ambient temperature affected the timing of fattening in captives. There was a seasonal change in propensity to fatten noted in captives. The abrupt inception and termination of prehibernatory fattening and the rapidity with which it occurs indicate that the response is due to hypothalamic hyperphagia and not to seasonal changes in food availability or to decreased metabolic rate. The response may be a manifestation of an annual rhythm in appetite that is phased periodically by environmental factors.