To investigate the effects of genotypes of dam and type of mating of parents on the litter sizes at birth and weaning, the dta collected from 3lines of mice and their crosses were analyzed. Maternal heterosic effects on the litter size were high in the crosses involving highly inbred C₃H line. The percent heterosis on the litter size was generally higher at weaning than at the time of birth which implies crossbred clams raise their pups better than straightbred dams. Back crosses exerted higher heterosis when the dams were younger (at early parity), while the 3-way crosses showed higher at older ages especially for the litter size at weaning.