It is a common knowledge that metabolic reactions increase in hyperthyroidism and decrease in hypothyroidism. The aim of this work was to investigate how the metabolic reactions could affect the total antioxidant status (TAS), protein concentration (PC) and the activities of acetylcholinesterase (AChE), (Na+,K+)-ATPase and Mg2+ -ATPase in the brain of hyper- and hypothyroid adult male rats. Hyperthyroidism was induced in rats by subcutaneous administration of thyroxine (25 microg/l00 g body weight) once daily for 14 days, while hypothyroidism was induced by oral administration of propylthiouracil (0.05%) for 21 days. TAS, PC, and enzyme activities were evaluated spectrophotometrically in the homogenated brain of each animal. TAS, PC, and Mg2+ -ATPase activity were found unaffected in hyperthyroidism, while AChE and Na+,K+ -ATPase activities were reduced by 25% (p < 0.01). In contrast, TAS, (Na+,K+)-ATPase and Mg2+-ATPase activities were found to be increased (approx. 23-30%, p < 0.001) in the hypothyroid brain, while AChE activity and PC were shown to be inhibited (approx. 23-30%, p < 0.001). These changes on brain enzyme activities may reflect the different metabolic effects of hyper- and hypothyroidism. Such changes of the enzyme activities may differentially modulate the brain intracellular Mg2+, neural excitability, as well as the uptake and release of biogenic amines.