The concentrations of adrenaline, noradrenaline, dopamine, aldosterone, the atrial natriuretic hormone, and plasma renin activity were investigated in 50 patients with mild chronic heart failure. The patients received oral digoxin chronically in a daily dose of 0.125 mg. On the basis of the estimate of the dosing of digoxin these patients were divided into two groups: the first with therapeutic and the second with subtherapeutic concentrations of digoxin in serum. The therapeutic concentration of digoxin in serum was found in 23 patients (46%), while subtherapeutic levels were found in 27 patients (54%). The concentrations of noradrenaline, dopamine, the renin activity of plasma, aldosterone and the atrial natriuretic hormone in the blood serum in the group of patients in whom the presence of subtherapeutic concentrations of digoxin was found, did not differ essentially from the concentration that was observed in the group with therapeutic concentrations. Only the concentration of adrenaline was higher (p < 0.05) in the group of patients with therapeutic concentrations of digoxin. The above results reveal that the neuroendocrine activity of plasma (except for the concentration of adrenaline) is alike in both ranges of digoxin concentrations in serum.