We examined the effects of maternal age and parity on litter size in 2 populations of white-footed mice (Peromyscus leucopus) in southwestern Ontario, Canada, to determine whether these factors cause seasonal variation in litter size. Litter size increased from 1st to subsequent litters among females that bred for the 1st time in their natal year, but not among females that 1st bred as overwintered adults. Thus, prior reproductive experience was not an important determinant of litter size. Maternal age accounted for approximately 70% of the variance in litter size; mean litter size was greatest among females between 150 and 250 days of age at parturition. Date of birth explained 79% of the variation in mean litter size, with litters born in summer significantly smaller than those born in spring or autumn. This effect was attributed to the fact that most summer litters were produced by primiparous young-of-the-year females. Although litter size declined among old, multiparous females, few individuals survived to the age at which reproductive senescence was apparent. Thus, reproductive senescence plays a minor role in population-wide variation in litter size.