Body composition, caloric value, and size of selected organs of arctic ground squirrels (Spermorphilus parryii) were examined seasonally and in relation to reproductive status in order to determine if females obtain nutrients for offspring from stored reserves prior to the availability of new vegetation. Seasonal changes in body composition were similar in males and females. Females showed a depletion of skeletal components, total fat, and caloric value during lactation. The mean reduction in caloric value between pregnancy and lactation (495 kcal; 1 cal = 4.1868 J) accounted for only 31% of the estimated 1600 kcal contributed to offspring by a female. Females must support offspring through an increased ingestion of food prior to the availability of new vegetation.