Inborn errors of lipid metabolism illustrate the importance of proper milk fat oxidation in newborn mammals. In the liver, a remarkable lipid catabolic competence is present at birth; however, it is unclear how this critical trait is acquired and regulated. In this work, we found that the genes required for milk lipid catabolism are already transcribed before birth in the term fetus (E19.5) and controlled by the peroxisome-proliferator activated receptor alpha (PPARα) in mouse liver. The developmental activity of PPARα strongly regulates fatty acid oxidation genes. Two days after birth (P2), during milk suckling, PPARα-null mice develop a congenital steatosis and milk protein oxidation is de-repressed to fuel an alternative energy pathway that maintains glucose homeostasis and postnatal growth. Our results demonstrate for the first time, the developmental role of PPARα in regulating the metabolic ability to use maternal milk as fuel in the early days of life. Expression profile difference between PPARalpha wild-type (3 males and 3 females) and knock-out (3 males and 3 females) mouse liver in suckling mice. Two days after birth, pups were killed and their liver promptly removed and frozen in liquid nitrogen.