Peak milk yield in the ruminant animal is attained relatively early during lactation, and this is followed by a progressive decline so that at the time of "drying off" yield may be reduced to only 50% of its maximum value. Studies in lactating goats indicate that the fall in milk yield after peak was due ultimately to a decrease in the number of secretory cells and that the remaining cells did not lose their metabolic capacity for milk synthesis to any significant degree. Manipulation of milking frequency or efficiency during declining lactation had both acute and long-term effects on milk yield and mammary cell number and activity. Rapid and reversible stimulation of yield by thrice daily or hourly milking was due to more frequent removal of a whey protein, which modulates milk secretion by negative chemical feedback on the secretory cell. Evidence is presented that the feedback inhibitor may act primarily on the secretory process rather than through inhibition of synthesis of individual milk constituents. Long-term thrice daily milking increased milk yield in the later stages of lactation by preventing, at least in part, the fall in secretory cell number after peak. Secretory cell differentiation was also susceptible to manipulation in declining lactation: inefficient milking for 24 wk after peak, i.e., by increasing the residual milk volume in the gland after milking, caused partial involution (dedifferentiation) of the existing secretory cell population. The role of local intramammary mechanisms in modulating milk yield and mammary function in declining lactation is discussed.