Timing of activity, seasonal changes in food availability, environmental fluctuations, and costs of reproduction are important determinants of an animal's life history. In mountain ground squirrels (Spermophilus), the shortness of the active season influences how they time their activities. I studied timing of the active season, reproduction, and weight loss during hibernation in Columbian ground squirrels (S. columbianus) in Alberta, Canada, from the summer of 1994 to the spring of 1998. Entry into hibernation and emergence from hibernation the following year were earlier for females that did not wean a litter compared with those that did. The active season was shorter and hibernation longer for unsuccessful females than for successful females. Molt of females was earlier when they did not wean a litter than when they did. Date of mating had no influence on success in weaning a litter, but juveniles surviving to yearling age came from litters born earlier than juveniles that did not survive to yearling age. In successfully reproducing females, early entry into hibernation was associated with early mating. Daily and total weight loss during hibernation was higher in females that weaned young than in females that did not wean young.