Seven cows were each milked at a variety of intervals between milkings a t each of several stages of lactation, intervals being long enough for maximum attainable yields to be approximated. The udder was emptied thoroughly at the beginning and end of each interval with the aid of injections of posterior pituitary extract. Intra-mammary pressure was estimated at. each milking. Net capacity of the udder, as measured by maximum yield, declined during lactation in proportion to the concurrent decline in daily yield, so that any given long interval between milkings reduced yield to almost the same degree in late lactation as in early lactation. The decline in maximum yield or "physiological capacity" was contributed to jointly by decline in 'physical capacity' (volume contained at given intra-mammary pressure) and decline in maximum pressure or secretion pressure. In early lactation, decline in secretion pressure contributed most to decline in physiological capacity, loss of physical capacity becoming important only in late lactation. It appeared that high rate of physical involution was associated with low functional activity of secretory tissue, as measured by secretion pressure, rather than with advanced lactation per se. It is postulated that the physical and functional components of physiological capacity are paralleled by physical and functional components of daily yield, namely, amount of secretory tissue and secretion rate per unit of tissue. Accordingly, decline in secretory intensity contributes primarily to early decline in daily yield, loss of secretory tissue becoming important in late lactation or in cows in which functional activity of secretory tissue is low. Cows appear to differ in the relative contributions of mammary development and functional activity to lactational performance. The pattern and course of involution during lactation may be partly determined by these initial differences. The effect of length of interval between milkings upon secretion is discussed, with emphasis on the difference between immediate and long-term effects. It is pointed out that differences between cows in sensitivity of secretion to long intervals are morphologically rather than functionally determined.