The norepinephrine content of hypothalamus in vehicle-treated control rats gradually increases between days 16 and 45 of life. Norepinephrine contents of the hypothalamus and mesencephalon of the thyroxine-treated (10 µg/g b.w., s.c., days 8–10) animals also increase between days 16 and 21. Mesencephalic and hypothalamic dopamine contents show no definite patterns throughout this period. The norepinephrine content of both regions is higher in the thyroxine-treated than in control animals between days 16 and 21. Hypothalamic dopamine content is higher and mesencephalic dopamine is lower in thyroxine-treated than in control animals between days 16–21. Corticosterone treatment (20 µg/g b.w., i.p., days 2–4) results in lower norepinephrine content of both regions, lower dopamine content of hypothalamus and higher dopamine content of mesencephalon between days 16 and 21. Thyroxine treatment (days 8–10) counteracts all these effects of the corticosteroid in animals receiving both treatments. Thus, neonatal thyroxine and/or corticosterone treatment significantly affect the developmental pattern of hypothalamic and mesencephalic catecholamines.