In victims of Sudden Infant Death Syndrome, the activity of phenylethanolamine-N-methyltransferase was found to decrease in the nucleus retroambigu, the Kolliker-Fuse nucleus and the C 2 area of medulla, whereas a decrease in the dopamine-beta-hydroxylase activity was found only in the C 2 area. These results suggest a hypoactivity of adrenergic neurons in respiratory and cardiac centres in the Sudden Infant Death Syndrome.