Carbonic anhydrase (CA) and Mg2(+)-dependent ATPase and Mg2(+)-dependent, HCO3(-)-stimulated ATPase (Mg2(+)-HCO3(-)-ATPase) activities in rat duodenal mucosa and kidney cortex were examined with respect to thyroidal status. Administration of 50 and 150 micrograms thyroxine (T4)/kg per day s.c. for 7 days decreased duodenal cytosol CA activity to 66% of control with the former and 43% with the latter dose, while Mg2(+)-HCO3(-)-ATPase activity in brush borders of duodenal mucosa was increased to 116% of control by 150 micrograms T4/kg. CA and Mg2(+)-HCO3(-)-ATPase activities in the cytosol and brush border of kidney cortex did not change after administration of T4. Hypothyroidism induced by thyroidectomy for 2 and 4 weeks or administration of methimazole (2.5-20 mg/kg per day s.c. or peroral) for 2, 3 and 4 weeks all increased duodenal cytosol CA activity, to about 140% at 2 weeks and 153% at 4 weeks after thyroidectomy, and to about 136% after the oral administration of 10 mg methimazole/kg per day for 4 weeks, while brush border Mg2(+)-HCO3(-)-ATPase activity was decreased to 56% of control 4 weeks after thyroidectomy and to 74% after the s.c. administration of 20 mg methimazole/kg day for 3 weeks. The increase in CA activity and the decrease in ATPase activity after thyroidectomy were restored to normal levels by replacement with T4. Neither enzyme activity in the kidney changed in hypothyroidism. Serum concentrations of T4 and cortisol-like material increased after administration of T4, and serum concentrations of T4, aldosterone and cortisol-like material all decreased in hypothyroidism. Correlations were observed between duodenal CA and Mg2(+)-HCO3(-)-ATPase activities and serum concentrations of T4 (P less than 0.01). These results reveal that the decrease in CA activity and the increase in Mg2(+)-HCO3(-)-ATPase activity of duodenal mucosa in hyperthyroidism are reversed in hypothyroidism, while both enzyme activities in the kidney are unrelated to thyroidal status.