Using deoxyribonucleic acid (DNA) as index of mammary gland growth, a study of changes from virginity throughout pregnancy and lactation in Webster-Swiss mice was conducted. Previous morphological studies were interpreted as indicating complete lobule-alveolar development by the first 1/2-2/3 of pregnancy. Our data show, however, only 30.6% of total growth during first 12 days of pregnancy, 45.7% in latter half of pregnancy and 21.9% further proliferation from parturition to day 14 of lactation. Growth of the gland continues throughout pregnancy, but in pseudopregnancy it is completed by day 6. The DNA level of ovariectomized lactating mice is similar to normal lactators, indicating that the corpora lutea of lactation does not contribute to additional growth of the mammary gland.