Seventy-six patients (6 males and 70 females) with diffuse toxic goiter, stages I-II, received lithium carbonate as a thyrostatic drug. The drug dose ranged from 900 to 1500 g depending on the degree of the disease clinical symptoms. The treatment with lithium lasted 45 days. Before drug administration and on days 7, 15, 30 and 45 of treatment the content of triiodothyronine (T3), thyroxine (T4) and calcitonin was measured in the thyroid, that of parathyroid hormone (PTH) in the parathyroid gland, and that of thyrotropic hormone (TTH) in the pituitary. The concentration of ionized calcium in the serum, calcium excretion with urine, and tubular calcium reabsorption were measured concurrently. In patients with diffuse toxic goiter treated with lithium, calcium excretion with urine substantially reduced, whereas tubular reabsorption of calcium and phosphates increased. However, serum calcium concentration did not rise, remaining within normal during all the treatment periods. In the author's opinion, this was favoured by two factors: the lithium-induced increase in interstitial calcium absorption on the one hand and compensatory increase in PTH secretion on the other one. The decreased content of thyronines in the hemocirculation (T3, T4), a short-term elevation of TTH and calcitonin elevation in the blood and steady increase in PTH secretion were characteristic features of the time course of the hormonal parameters in patients with toxic goiter treated with lithium.