Summary The potency and output of carotene, vitamin A, and riboflavin in the colostrum and milk of 35 pasture-fed cows representing five dairy cattle breeds were determined for the first ten milkings and the twentieth milking. Cows of all ages were included. The data obtained show a rapid decline in the concentration of these constituents with successive milkings. A level closely approaching that of normal milk is reached by the end of the third day following parturition (sixth milking). These data show first milking colostrum to be approximately ten times as potent in carotene, six times as potent in vitamin A, and 3.3 times as potent in riboflavin as normal milk obtained at the twentieth milking. The practical importance of using all the colostrum produced in the raising of calves is pointed out.