In late fall and winter, Great Basin pocket mice remain in their burrows in a state of torpor. They emerge from their burrows and mate in early spring. Males emerge slightly before females. In south-central Washington, Great Basin pocket mice emerged from March to April. Prebreeding enlargement of ovaries and testes begins in winter in the complete darkness of the burrow. Following emergence from the burrow, the lengthening photoperiod of spring apparently triggers final enlargement and development of gonads for breeding. Access to succulent green vegetation in spring may enhance reproductive success of females. Captive female Great Basin pocket mice from eastern Washington fed lettuce and seeds had significantly larger ovaries than control females fed only seeds. Great Basin pocket mice remain reproductively active through summer. Females produce one or two litters per year. Most first litters are delivered in May and second litters in August. Reports of average litter size have ranged from 3.9 in south-central Washington to 5.6 in Nevada. First-litter subadults first leave the natal burrow in early summer; subadults from the second litter first emerge in fall. In a 2-year