Changes in the norepinephrine concentration in the hypothalamus, amygdala, hippocampus, and cerebral cortex with age were studied. Female rats were sacrificed at 10, 20, 30, 36 or 37, and 51 days of age. The 36- and 37-day-old rats were divided into animals with opened vaginas and animals with closed vaginas. Tissue norepinephrine concentration was determined by the trihydroxyindole fluorescence technique. The pattern of the ontogeny of norepinephrine in the four areas studied was similar. Adult levels of the amine were observed in the hypothalamus well before the onset of puberty. No change in the concentration in the hypothalamus or amygdala occurred concurrent with vaginal opening. However, a drop in norepinephrine concentration in the hypothalamus occurred between 30 and 37 days of age.