Milk is essential to a mammalian mother's reproductive strategy and is necessary for offspring growth and development. In hibernators with a short duration between weaning and winter immergence, milk synthesis is likely constrained by time and trade-offs between maternal and offspring condition, thus influencing milk composition. We characterized the proximate and mineral composition of milk produced by a hibernating rodent, the Columbian ground squirrel (Urocitellus columbianus). The concentration of all milk components varied across lactation; the concentration of most constituents peaked between days 14 and 19 postpartum. Columbian ground squirrel milk was relatively low in lipids but high in protein and calcium. At peak lactation, milk was composed of 10.71% ± 0.46% SE protein, 9.15% ± 0.47% lipids, 3.39% ± 0.13% sugar, and 0.47% ± 0.02% calcium (wet mass basis). High protein, energy from protein, and calcium in milk corroborate earlier reports of the importance of fast growth rates of juveniles to overwinter survival, whereas the low lipid content of milk may reflect fat conservation for adults. Production of high-calcium milk also may be a preventive mechanism enabling offspring to cope with bone mineral loss during hibernation.