Studies on increasing the frequency of milking in dairy animals have led to the uncovering of the mechanism by which tactical control of the rate of milk secretion is achieved locally within each mammary gland, against a strategic, systemic control by the hormones that maintain all glands in the secretory condition. Experiments in vivo established that the response is local, and were compatible with the hypothesis that milk contains an inhibitor of its own secretion which accumulates during storage within the lumen of the mammary gland and which acts in an autocrine manner on the secretory cells. Isolation of a protein, initially from goats' milk, called FIL (feedback inhibitor of lactation) has enabled, and is enabling, further studies to be done from the whole-animal down to the molecular level. Examples at the whole-animal level are: the effects of immunization against FIL on the rate of secretion; the concentration of FIL and the kinetics of its formation and breakdown; the importance of the internal structure of the mammary gland and the capacities of the alveolar and ductular storage regions in determining feedback inhibition; differences between individuals and species influencing the degree of control exerted by FIL in matching supply of milk to demand by the young. Other local control mechanisms at the onset and cessation of lactation, including mammary distension, are also discussed.