In this second part of our two-part commentary, milk production and milk synthesis are defined and the methods that may be used in their measurement are briefly reviewed. In particular, the rationale for the development of breast volume measurement techniques is described. We review our studies which have employed breast volume measurement techniques and propose a model for the short-term control of milk synthesis in lactating women. According to this model, the breast responds to the degree to which the infant empties the breast at each breastfeed. The frequency of milk removal may not directly affect the rate of milk synthesis except as a function of the mother's ability to store milk.