Males become sexually active in May or June, and stay that way until September. Females are receptive in May and June. They give birth in July or August, with a gestation period of 18-23 days. There is one annual litter of 4-8 young. The altricial newborn is pink and hairless; its eyes are shut, its ears is folded, its head is short and stubby, and its facial vibrissae are not yet visable. This tiny newborn weighs .7-.9 grams. It is weaned after four weeks. It will become independent at around one month, and sexually mature the next year. The Pacific jumping mouse can reach a maximum age of at least four years in the wild.
(Gannon 1988, Niethammer 1990)