Quantitative measurements were made of relative breast volume and milk production from 1 month of lactation until 3 months after weaning, and the storage capacity of the breasts was calculated. The increase in breast tissue volume from before conception until 1 month of lactation was maintained for the first 6 months of lactation (means +/- S.E.M.) (190.3 +/- 13.1 ml, number of breasts, n(b) = 46). During this period of exclusive breast-feeding, 24 h milk production from each breast remained relatively constant (453.6 +/- 20.1 g, n(b) = 48), and storage capacity was 209.9 +/- 11.0 ml (n(b) = 46). After 6 months, breast volume, milk production and storage capacity all decreased. There was a relationship between 24 h milk production and the storage capacity of the breasts, and these both appeared to be responding to infant demand for milk. At 15 months of lactation, the 24 h milk production of each breast was substantial (208.0 +/- 56.7 g, n(b) = 6), even though the breasts had returned to preconception size. This was associated with an apparent increased efficiency of the breast (milk production per unit breast tissue) after 6 months, which may have been due to redistribution of tissues within the breast. The possible causes of the decrease in breast volume are discussed.