In the female catfish Heteropneustes fossilis, administration of thyroxine (T(4))(,) 1 micro g/g body weight, i.p., in both gonadal resting and preparatory phases for 7, 14 and 21 days caused hyperthyroidism, as evidenced from a duration-dependent significant increase in serum triiodothyronine (T(3)), and of tyrosine hydroxylase (TH) activity in telencephalon, hypothalamus-pituitary and medulla oblongata (Newman-Keuls' test; P<0.05). Hypothyroidism induced by adding 0.03% thiourea to aquarium water holding the catfish for 7, 14 and 21 days decreased serum T(3) levels in a duration-dependent manner (Newman-Keuls' test; P<0.05) and inhibited TH activity in the brain regions. T(4) replacement in 21day thiourea-treated fish restored and even elevated significantly serum T(3) levels as well as brain TH activity in a duration-dependent manner. In general, the changes in enzyme activity were higher in the forebrain regions than medulla oblongata and in the resting phase than preparatory phase. Kinetic studies by Lineweaver-Burk plots showed that the stimulatory effect following T(4) administration and T(4) replacement on TH activity was due to increased affinity of the enzyme for its cofactor (6,7-dimethyl-2-amino-4-hydroxy-5,6,7,8-tetrahydropteridine), as evident from a significant decrease in apparent Michaelis-Menten constant (K(m)) and an increase in apparent velocity maximum (V(max)). The TH inhibition due to the thiourea treatment can be related to decreased affinity of the enzyme for its cofactor, as evident from a significant increase in apparent K(m) value and a significant decrease in V(max). These data clearly show that circulating levels of T(4)/T(3) modulate brain TH activity by altering the kinetic properties of the enzyme, which, in turn, influence catecholaminergic activity and dependent functions.