A cross-fostering experiment was conducted on two quite distinct subspecies of mice, domesticated laboratory mouse of CF#1 (Mus musculus domesticus) and Yonakuni wild mouse (Yk, Mus musculus molossinus yonakuni), to estimate the prenatal and postnatal maternal effects on body weight of offspring. Mating was done between subspecies, two or three females being mated to a male at nine-ten weeks of age. Two dams of different subspecies that littered at the same day were used as a group of foster dams. Litters were standardized to six young mice in order that a dam nursed three mice of her own litter and three mice from that of another subspecies dam. The litters were weaned at 3 weeks of age. The body weight of individual mice was determined at 1, 3, 6 and 10 weeks of age. The result demonstrated that prenatal maternal effects were more important than postnatal maternal effects in contributing to the variation in body weight at all ages examined. Prenatal maternal effects accounted for 61-96% and 35-92% of total variance in males and females, respectively; whereas postnatal effects accounted for 1-7% for males and 3-23% for females. Analysis for between postnatal within prenatal, and between prenatal within postnatal indicated that expression of the body weight of offspring was limited by the genetic type of their prenatal dam and influenced by the postnatal environment of nursing dam. The greatest body weight was attained by offspring born to prenatal CF#1 dams and nursed by postnatal CF#1 dams, followed by CF#1 offspring born to CF#1 dams and nursed by Yk dams, Yk offspring born to Yk dams and nursed by CF#1 dams and the lightest ones were Yk offspring born to Yk dams and nursed by Yk dams.