Breast volume and morphology of eight subjects were measured before conception and at intervals throughout pregnancy until 1 month of lactation. Breast volume before conception ranged from 293 to 964 ml. At the end of pregnancy the volume of breast tissue had increased by 145 ± 19 ml (mean ± S.E.M., n = 13 breasts, range 12-227 ml) with a further increase to 211 ± 16 ml (n = 12 breasts, range 129-320 ml) by 1 month of lactation. Urinary excretion of lactose increased at 22 weeks of pregnancy, signalling the capacity of the breast to synthesize lactose at this time. During pregnancy, both the change in breast volume and the change in cross-sectional area of the areola were related to the concentration of human placental lactogen in the plasma. The growth of the nipple and the rate of excretion of lactose were related to the concentration of prolactin in the plasma. During the first 3 days after birth, the rate of excretion of lactose was related to the rate of excretion of progesterone. There was no relationship between the growth of the breast during pregnancy and the amount of milk produced at 1 month of lactation.