Abstract
SHN/Mei female mice were classified into 8 groups according to their lactation records in 3 litters from group 1 with successful lactation in all 3 litters to group 8, which failed to lactate in all litters, and the differences in reproductive parameters and the age of onset of mammary tumours was compared between group 1 and the other groups. Pup growth was retarded and the age of onset of mammary tumours were shortened by the experience of the unsuccessful lactation; both parameters were largely affected by the proliferation and differentiation of mammary glands, which are modulated by the unsuccessful lactation. The unsuccessful lactation did not affect consistently litter size or rearing rate.