Milk yield is greatly influenced by the size of the mammary secretory cell population. In rats, proliferation of cells during early lactation may account for as much as three-quarters of the increase in daily milk yield between parturition and peak lactation; the remainder is due to increased activity of existing cells. Conversely, in goats, all the initial decline in milk yield after peak lactation can be attributed to loss of secretory cells. Increased frequency of milking enhances milk yield and reduces secretory cell loss, whereas goats hemimastectomized at peak lactation undergo compensatory changes in the remaining gland, which include a complete maintenance of cell number for at least 18 wk. Cell proliferation is increased in both cases, showing that mammary growth can occur during established lactation. The significance of this observation is discussed in relation to ways in which the milk yield of dairy animals might be increased.