Animals undernourished before birth and therefore born small are not particularly hungry and do not take enough food to enable them to catch up their larger fellows after birth even when there is a plentiful supply of milk. Animals and children undernourished after birth are very hungry and they eat far more per unit body weight than well-nourished individuals as soon as food is available. They gain weight rapidly for a time, and the ‘catch-up’ may or may not be complete depending on the severity and duration of the undernutrition. This difference in response to rehabilitation after undernutrition at different ages is attributed to the stage of development the appetite centres in the hypothalamus have reached when the undernutrition was imposed. Whether the animals undernourished after birth catch up in weight or not, they deposit more fat than normal, suggesting that appetite has outrun the ability to lay down lean tissue.