Activities of carbonic anhydrase and Na+,K(+)-ATPase in tissue homogenates and in subcellular fractions from different brain regions were studied in inherited primary hypothyroid (hyt/hyt) mice. The body weight, the weight of different brain regions, and the plasma thyroxine and triiodothyronine levels of hyt/hyt mice were significantly lower than those of the age-matched hyt/+ controls. In tissue homogenates of cerebral cortex, brain stem and cerebellum of hypothyroid mice, the activity of carbonic anhydrase (units/mg protein) was 59.2, 57.6, and 43.2%, and the activity of Na+,K(+)-ATPase (nmol Pi/mg protein/min) was 73.7, 74.4 and 68.7%, respectively, of that in corresponding regions of euthyroid littermates. The decrease in enzyme activity in tissue homogenates was also reflected in different subcellular fractions. In cerebral cortex and brain stem, carbonic anhydrase activity in cytosol, myelin and mitochondrial fractions of hypothyroid mice was about 45-50% of that in euthyroid mice, while in cerebellum the carbonic anhydrase activity in these subcellular fractions of hyt/hyt mice was only 33-38% of that in hyt/+ controls. Na+,K(+)-ATPase activity in myelin fraction of different brain regions of hyt/hyt mice was about 34-42% of that in hyt/+ mice, while in mitochondria, synaptosome and microsome fractions were about 44-52, 46-53, and 66-68%, respectively of controls. These data indicate that the activity of both carbonic anhydrase and Na+,K(+)-ATPase was affected more in the myelin than other subcellular fractions and more in the cerebellum than cerebral cortex and brain stem by deficiency of thyroid hormones.(ABSTRACT TRUNCATED AT 250 WORDS)