The aim of this study was to investigate the effect of AT1 -receptor antagonist, losartan usage in place of angiotensin converting enzyme (ACE) inhibitor, enalapril, in patients with congestive heart failure (CHF) on serum digoxin concentrations. The study was performed in 17 patients (14 men, 3 women) with CHF of New York Heart Association (NYHA) functional class II. Their ages ranged from 42-73 (61.9±2.4) years and they all were being treated with enalapril and digoxin at the time of the study. Serum digoxin levels were measured after 15 days of enalapril treatment just before taking digoxin dose in the morning and 2nd and 6th hours after digoxin medication. Then, losartan was added to treatment instead of enalapril and after 15 days, serum digoxin levels were measured again in the same patients. Trough serum digoxin levels were 1.19±0.09 ng/mL after 15 days of enalapril treatment and 1.30±0.13 ng/mL after 15 days of losartan treatment, the digoxin levels at two and six hours after the morning dosage were 1.79±0.18 and 1.43±0.13 ng/mL after enalapril treatment, and 1.88±0.16 and 1.37±0.12 ng/mL after losartan treatment. Losartan treatment did not change the serum digoxin concentrations compared to enalapril treatment. Losartan usage in place of enalapril in patients with CHF receiving enalapril together with digoxin did not affect serum digoxin concentrations.