The daytime activity of N-acetyltransferase per mg epiphysis decreased to 1/10 the newborn level by the age of 13 days, and subsequently remained unchanged. The night activity was equal to the day activity at the age of 3 days, was higher at the age of 6 days and was 20 X that of the day level at the age of 40 days. Keeping animals in constant light after birth depressed the development of these diurnal differences in N-acetyltransferase activity and slowed down the decrease in enzyme activity after birth. Sympathectomy with 6-hydroxydopamine after birth abolished the development of the diurnal rhythm in N-acetyltransferase in 12-day-old rats in two experiments out of five and only decreased the night activity without abolishing the rhythm in three experiments. Monoamine oxidase activity (MAO) per mg epiphysis, which is the same in newly born and in adult animals, decreased to half the original value for 6 days after birth and then increased again. Constant light after birth did not influence MAO activity, but sympathectomy with 6-hydroxydopamine decreased activity in the epiphysis in 12-day-old animals.