Mammalian herbivores from northern environments often store fat in summer to decrease the energetic stress of winter. We measured mass gain during the spring-to-fall season in a Quebec population of North American porcupines (Erethizon dorsatum) and determined how it is affected by the age, sex, and reproductive status of individuals. Porcupines gained mass without interruption from midspring to late summer, but mass gains were more important in mid- to late spring and late summer than at other times. This likely reflected local variations in the nutrient content of forage. Adult males lost mass during the rutting period of early fall. All age–sex classes except lactating females gained about 3 kg, or 40% of their spring body mass. This is not markedly different from mass gains observed in other nonhibernating mammalian herbivores. We generate the hypothesis that in this population some ecological, behavioral, or physiological mechanism limits the rate of mass increase to about 21 g per day.