Sir Frederick Gowland Hopkins took two sets of eight young rats and fed both sets a diet consisting of purified casein, starch, sucrose, mineral salts, lard and water. In addiction, one set was given 3 cm3 of milk every day for the first 18 days of the experiment. Milk contains vitamins. After 18 days the milk supplement was withdrawn from the first set and given to the second set of young rats until the end of the experiment. The results demonstrate the importance of vitamins in the growth rate of young rats.