We studied how short-term milk synthesis responds to milk removal by observing breast volume before and after each breastfeed over 24 h periods within the homes of seven lactating mothers, using the Computerized Breast Measurement system. Short-term rates of milk synthesis varied markedly between the breasts of individual mothers, varied markedly between interfeed intervals for individual breasts and, for six of the thirteen breasts studied, were positively related to the degree to which the breast was emptied (r2 ranging from 0.32 to 0.95). In addition, the infants rarely emptied the breasts of available milk (mean +/- S.D. of degree of emptying postfeed = 76 +/- 20%, n = 147) and, within women, storage capacity of a breast was related to the demand for milk from that breast (r2 = 0.91, P < 0.0001). It was concluded that the infants were self-regulating their milk intake. Furthermore, the storage capacity of the breast, variations in the short-term rates of milk synthesis and responsiveness of milk synthesis to the degree of breast emptying provided mechanisms whereby maternal milk supply could be directly linked to infant demand. Our findings are discussed with reference to the autocrine control of milk synthesis.