Effects of thyroid status on brain catecholamine turnover in adult rats were investigated using a steady-state method. Rats were treated for 3 weeks with s.c. injections of L-thyroxine (0.4 mg/kg), aminotriazole in drinking water (0.1%, w/v) or vehicle. After 2 weeks of treatment rats were implanted chronically with lateral intracerebroventricular (i.c.v.) cannulae. They were injected i.c.v. with [3H]tyrosine 1 week later. Catecholamine and tyrosine content and specific activity were measured in mediobasal hypothalamus, anterior hypothalamus and striatum, using high-performance liquid chromatography with electrochemical detection. Thyroxine treatment resulted in a significant increase in noradrenaline and dopamine synthesis localized to the mediobasal hypothalamus. Conversely, aminotriazole treatment resulted in a significant decrease in noradrenaline synthesis localized to the mediobasal hypothalamus. The localization of these changes in catecholamine turnover to the mediobasal hypothalamus suggests that they may be specific functional effects which are of importance in the overall integrated control of thyroid function.