In order to investigate the changes in the expression of extracellular signal regulated kinase (ERK1/ERK2) and angiotensin converting enzyme (ACE) in the patients with atrial fibrillation (AF), 52 patients with rheumatic heart diseases were examined. Nineteen patients had chronic persistent AF (AF≥6 months, CAF), 12 patients had paroxymal AF (PAF) and 21 patients had no history of AF. The ERK expression was detected at the mRNA level by reverse transcription polymerase chain reaction, at the protein level by Western blotting and at atrial tissue level by immunohistochemistry. ERK activating kinases (MEK1/2) and ACE were determined by Western blotting techniques. The expression of ERK2 mRNA was increased in the patients with CAF (74±19 U vs sinus rhythm: 32±24 U, P <0.05). Activated ERK1/ERK2 and MEK1/2 were increased to more than 150 % in the patients with AF compared to those with sinus rhythm. No significant difference between CAF and PAF was found. The expression of ACE was three fold increased in the patients with CAF compared to those with sinus rhythm. Patients with AF showed an increased expression of ERK1/ERK2 in atrial interstitial cells and marked atrial fibrosis. An ACE dependent increase in the amounts of activated ERK1/ERK2 in atrial interstitial cells may be one of molecular mechanisms for the development of atrial fibrosis in the patients with AF. These findings may have important impact on the treatment of AF. Nine of the patients had chronic persistent diseases (ACE) in the patients with atrial fibrillation (AF), 52 patients with rheumatic heart diseases were examined. AF (AF≥6 months, CAF), 12 patients with paroxymal AF (PAF) and 21 patients had no history of AF. The ERK expression was detected at the mRNA level by reverse transcription polymerase chain reaction, at the protein level by Western blotting and at atrial tissue level by immunohistochemistry. ERK activating kinases (MEK1 / 2) and ACE were determined by Western blotting techniques. The expression of ERK2 mRNA was increased in the patients with CAF (74 ± 19 U vs. sinus rhythm: 32 ± 24 U , P <0.05). Activated ERK1 / ERK2 and MEK1 / 2 were increased to more than 150% in the patients with AF compared to those with sinus rhythm. No significant difference between CAF and PAF was found. The expression of ACE w as three fold increased in the patients with CAF compared to those with sinus rhythm. Patients with AF showed an increased expression of ERK1 / ERK2 in atrial interstitial cells and marked atrial fibrosis. An ACE dependent increase in the amounts of activated ERK1 / ERK2 in atrial interstitial cells may be one of molecular mechanisms for the development of atrial fibrosis in the patients with AF.