The length of gestation and the duration of parturition were similar in mice that had mated during natural oestrus and in those with large numbers of full-term foetuses following superovulation. After superovulation many offspring die at parturition, but by giving every possible assistance to the offspring at birth, mean litter size was raised from 6·8 to 10·7 living young (controls that mated during natural oestrus gave birth to 6·2 living young). The mortality rate after birth was high in large litters, and offspring weighing less than 0·7 g at birth survived less than 24 hr. When one-half of the large litters which included offspring weighing 0·7 g or more were fostered on to control females, mortality was reduced to control levels and the offspring grew at the same rate whether a superovulated or control mother was suckling. The overall fecundity of superovulated mice was lower than controls because (a) 11 out of 137 of them died, (b) more than one-half of them failed to implant any embryos.