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In many patients operated on for valvular heart diseases indications are present to digitalis administration in the preoperative period. During extracorporeal circulation fluctuations occur of blood potassium concentration which increase the risk for toxic effect of digitalis. In the work, digoxin concentration was studied in the serum of patients operated on in extracorporeal circulation in whom administration of this drug was discontinued 48 hours before the operation. The first digoxin dose (0.5 mg) was given directly after withdrawal of extracorporeal circulation, and another 0.5 mg was given after 4 hours. Digoxin concentrations were determined by fluorescence-polarization-immunological method using TD* Abbott device. The digoxin concentration increased rapidly and reached its peak after 6 hours (1.9 ng/ml), decreasing after 12 hours to 0.9 ng/ml and persisted at this therapeutic level in further samples. The present studies demonstrated that digoxin administration directly after withdrawal of extracorporeal circulation and within 24 hours after the operation in commonly accepted doses provided therapeutic concentration of the drug and caused no increase of the risk for toxic reaction development.