The rate of milk secretion in farrow cows may be expressed as See PDF for Equation, in which y = yield and t = time from calving. Pregnancy causes a decrease in yield which may be expressed as See PDF for Equation, in which i = inhibition or decrease in yield and p = time from conception. The constant K appears to be the same for various groups but b is roughly proportional to a. The decrease in yield associated with pregnancy is interpreted as due to a hormone. The hormone hypothesis also affords an interpretation of the increasing rate of milk secretion which occurs for a short time following parturition.