The effect of the temporal spacing and degree of milk removal by either expression or breastfeed on short-term (hour to hour) rates of milk synthesis was investigated. For four expressing mothers (three of whom had given birth prematurely), the relationship between milk produced at an expression and the time since the last expression showed that proportionally less milk is produced after intervals of 6-18 h than after intervals of less than 6 h (P < 0.0002). For five breastfeeding and four expressing mothers (three of whom had given birth prematurely), rates of milk synthesis (ranging from 0 to 56 ml/h) were linear between breastfeeds and expressions (intervals ranging from < 1 h to 6 h). Changes in rate of milk synthesis were not directly associated with the frequency of breastfeeds but rather with the degree of emptying of the breast (P < 0.05). These results indicate the importance of degree, rather than frequency, of breast emptying in the short-term control of human milk synthesis.