The changes of phenylethanolamine-N-methyltransferase (PNMT, EC 2.1.1.28), the enzyme that catalyzes the final step in the biosynthesis of adrenaline, were studied during the development of several regions of rat brain. PNMT is present in medulla oblongata-pons, hypothalamus, cerebellum, and midbrain five days before birth, and a progressive increase in the enzyme activity is observed during development. The adult levels are attained between 15 and 20 days, depending on the region. The increases in PNMT activity in the rostral regions are higher than in the caudal regions. PNMT attains adult levels earlier than tyrosine hydroxylase and dopamine-beta-hydroxylase. The apparatus for adrenaline synthesis seems to be mature at three weeks after birth in the medulla oblongata-pons, which contains the cell bodies of adrenaline-containing neurons. In the other regions the adult levels of enzyme activity are attained at 15 days after birth.