This experiment establishes the importance of in utero litter size in shaping the bioenergetic characteristics of the adult female mouse. The potential number of fetuses carried by a pregnant female was reduced to half by blocking one oviduct prior to mating. The resulting offspring were reared in normal sized litters after birth. As adults these female offspring were heavier and they consumed less food than control females who had shared their mother's uterus with a normal complement of fetuses. As assessed with running wheels, females from reduced litters were less than half as active during adulthood as control females. Variation in prenatal litter size apparently has far reaching effects on the acquisition and use of energy by mice.