Using the Falck-Hillarp methodology in combination with quantitative microñuorimetry, catecholamine (CA) levels and utilization in discrete hypothalamic CA nerve terminal systems in the male rat have been analyzed 24 h, 1, 2 and 4 weeks following thyroidectomy as well as after T3 or T4 restitution therapy 4 weeks after thyroidectomy. By means of high pressure liquid chromatography (HPLC), dopamine (DA) and noradrenaline (NA) levels and utilization 4 weeks after thyroidectomy have been analyzed in various brain regions. Triiodothyronine treatment (10 µg/kg, s.c, twice daily during 10 days) of 4-week athyroidic rats increased serum T3 levels, but not T4 serum levels. Thyroxine treatment (36 µg/kg, s.c, twice daily during 10 days) of 4-week athyroidic rats increased serum T4 levels and the T3 levels were found to be even slightly higher than those found in normal animals. Triiodothyronine or T4 restitution therapy reversed the changes induced by thyroidectomy on the anterior pituitary hormones (TSH, prolactin and growth hormone) and corticosterone secretion. It is suggested that removal of thyroid hormones may be responsible for the changes in the anterior pituitary hormones and corticosterone secretions. In the quantitative microfluorimetrical analysis 24 h, 1,2 and 4 weeks after thyroidectomy, decreases in DA levels and utilization and increases in NA levels and utilization were found in the lateral palisade zone (LPZ) of the median eminence and in the parvocellular and magnocellular parts of the paraventricular hypothalamic nucleus (PA), respectively. In addition, 1-, 2- and 4-week decreases were found in DA levels and turnover in the medial palisade zone (MPZ) as well as in NA turnover in the dorsomedial hypothalamic nucleus (DM) and in the ‘border zone’ (BZ) between the medial and lateral hypothalamus ventral to the fornix. In the HPLC analysis it could be demonstrated that 4 weeks after thyroidectomy decreases in DA levels and utilization in the mediobasal hypothalamus and increases in DA levels as well as NA levels and utilization in the hypothalamus had developed. The T3 or T4 restitution therapies after 4 weeks of thyroidectomy counteracted the effects on CA levels and utilization in all hypothalamic CA nerve terminal systems except for the reduced NA utilization found in the DM following 4 weeks after thyroidectomy. The demonstrated reductions in DA levels and utilization in the MPZ and LPZ of the median eminence and the increases in NA levels and utilization in the PA are suggested to reflect the existence of an inhibitory dopaminergic mechanism in the median eminence and a facilitatory noradrenergic mechanism in the PA involved in the regulation of TSH secretion. These mechanisms seem to be directly or indirectly regulated by thyroid hormones. The reduced NA utilization in the DM and in the BZ after thyroidectomy is suggested to be related to the lowering of growth hormone secretion and reduction of its pulsatile release in this endocrine