en پژوهشي Research Article It has been reported that some of H1 receptor antagonists have important effects on cardiovascular system. Terfenadine as a non-sedative H1 receptor antagonist has an arrhythmogenic activity. In this study we have shown the effects of four antihistamine drugs: terfenadine, loratadine, clemastine and diphenhydramine, on the rate and contractions of isolated rat atria. Terfenadine (1-10 µM) caused a negative chronotropic effect (19.5-80%) and arrhythmia after 10 min. followed by a decrease in the contractile force by (7.5%), and finally after 45 min. asystolia occurred. Loratadine (30-150 µM) decreased the rate of contractions (10-82%) after 10 min. the contractile force of atria was decreased (10-19%) after 20 min. Loratadine did not produce any arrhythmia. Diphenhydramine (5-20 µM) produced bradycardia (14.5-43%) after 20 min and decreased the contractile force (2.5-40%) after 40 min. Clemastine (3-10 µM) produced negative chronotropic and inotropic effect by (11.5-42%) and (10-58%) respectively. These findings indicate that all four drugs caused bradycardia and reduced contractile force, but in the case of terfenadine, it also had arrhythmogenic activity. Loratadine had the least cardiotoxic effect. 20 0 http://ijpt.iums.ac.ir/browse.php?a_code=A-10-100-27&slc_lang=en&sid=1 Abbas Pousti 100319475328460056 100319475328460056 No Golrokh Malihi 100319475328460057 100319475328460057 No Azam Bakhtiarian 100319475328460058 100319475328460058 No Zahra Abdollahi 100319475328460059 100319475328460059 No