During the first 4 postnatal days, the atrioventricular specialized tissue of the rat contains butyrylcholinesterase alone. The next 7 days are associated with a mixture of both acetyl and butyryl activity, but after the 12th day, acetylcholinesterase is found to predominate largely. It is suggested that this change in activity is related to the growth of adrenergic nerves into the heart on the 4th day. Administration of antinerve growth factor prevents the development of these nerves and is found to delay the onset of the change in cholinesterase activity from butyryl to acetyl from the 5th day until the 21st. Only after the 31st day is acetylcholinesterase the most prominent enzyme in treated animals.