Cardiac side effects of beta 2 agonists have been evaluated in many studies. The most important factors were hypoxia and hypokalemia. We searched for the cardiac side effects of formaterol in asthma and COPD patients with Pa 02> 60 mm Hg, FEV1 > %50 and without any other cardiac disease. Patients after a period of withdrawal of bronc-hodilatator therapy were applied placebo, Formaterol 12 microgram and 24 microgram on days 0, 1 and 2 respectively. Their arterial blood gases, serum electrolytes and cardiac rithyms by Holter moniterisation were evaluatedfor three days. Serum potassium levels significantly decreased in correlation with formaterol dose. 12 microgram formoterol did not change Pa 02 levels compared with placebo though 24 microgram decreased Pa 02 significantly. Also 24 microgram caused a significant decrease of Pa 02 compared with 12 microgram (p<0.05). 24 microgram formaterol decreased serum potassium levels more than 12 microgram; but neither doses caused difference in atrial and ventricular arrythmia pruduction. We conclude that in mild to moderate stable asthma and COPD patients without hypoxemia both 12 and 24 microgram of formaterol did not cause significant arrythmia although they gave rise a decrease in potassium levels inaccordance with dose.