The author presents the results of parallel studies of potassium, sodium and calcium in the blood plasma and the 24-hour urine of 146 patients suffering from toxic goiter of different severity (mild course in 43 patients, of moderate severity in 59, and severe in 44). Investigations were carried out before and after the treatment with thyrotoxic preparation and in some of the patients (24) after strumectomy. Before the treatment patients with mild and moderately severe forms of the disease showed a tendency to increase of potassium and calcium and a fall of sodium level in the blood. Patients with a severe course of thyrotoxicosis displayed an opposite tendency in the changes of plasma electrolyte composition--the majority of the patients showed hypokaliemia, hypocalciemia and hypernatriemia. A tendency to normalization of plasma ionic composition was noted after the treatment with thyrotoxic preparations during the euthyroid state. Sodium and calcium plasma level became normal after strumectomy; potassium content remained lowered. Correction of electrolyte metabolism should be carried out in the patients with a severe form of toxic goiter; potassium and calcium administration is recommended.