Using catecholamine (CA) fluorescence histochemistry in combination with quantitative microfluorimetry, it has been shown that chronic treatment with triiodothyronine (T3) and thyroxine (T4; 2 × 10 and 2 × 36 µg/kg i.p., respectively, twice daily for 10 days), but not acute treatment (1 and 3.6 mg/kg i.p., respectively, 2 h before killing), increases CA utilization in the medial and lateral palisade zones of the median eminence and reduces noradrenaline (NA) utilization in the parvocellular part and magnocellular part of the paraventricular hypothalamic nucleus of the hypophysectomized male rat. Following chronic T4 treatment it could also be shown that the CA levels in the medial and lateral palisade zones of the median eminence were increased, while the NA levels were reduced in the parvocellular part of the paraventricular hypothalamic nucleus. Chronic T3 treatment induced similar changes – increased CA levels in the medial palisade zone and reduced NA levels in the magnocellular part of the paraventricular hypothalamic nucleus. Within the telencephalon, chronic but not acute treatment with T3 or T4 selectively increased dopamine (DA) utilization within the diffuse type of DA nerve terminal systems of the nucleus accumbens. This action of chronic treatment of T3 or T4 was highly selective and no changes in DA levels could be demonstrated in any DA nerve terminals analyzed in the nucleus caudatus putamen, nucleus accumbens and tuberculum olfactorium. In all the experiments the TSH levels remained undetectable and the low basal serum prolactin levels were not modulated in any experimental group in spite of the treatment with a tyrosine hydroxylase inhibitor in the CA utilization experiments. Following 2–3 weeks after hypophysectomy, serum T3 and T4 were decreased by 30–50%. In the acute experiments with T3or T4, serum T3 levels and T3 as well as T4 levels were markedly elevated after the respective treatments. In the chronic experiments, the T4 treatment resulted in significant increases in the serum levels of both T3 and T4. The present results indicate that discrete DA and NA nerve terminal systems within the median eminence (DA), nucleus accumbens (DA) and paraventricular hypothalamic nucleus (NA) can slowly respond to chronic treatment with T3 or T4. This effect is the result of a direct action of the thyroid hormones on the brain since TSH is absent in the hypophysectomized rat. The results indicate that thyroid hormones interact with those DA and NA nerve terminal systems known to be involved in the inhibitory dopaminergic mechanism in the external layer of the median eminence and also interact with the facilitatory noradrenergic mechanism within the parvocellular part of the paraventricular hypothalamic nucleus regulating TSH secretion. In view of the selective increase in DA turnover in the nucleus accumbens it is speculated that chronic thyroid hormone treatment may modulate behaviors by a direct central action. The effects of thyroid hormones in the CNS may be mediated via nuclear receptors. However, this remains to be demo