Levamisole is frequently used as an effective anthelminthic drug. It has a wide scope on the GI and respiratory system worms. It acts as a cholinergic receptor agonist and causes neuromuscular paralyses in target parasites. This drug may result in intoxication if is taken more than standard dose because of a limited safety margin. Poisonous dose is tenfold greater than the therapeutic dose. Unfortunately, levamisole has an unsuitable side effect on cardiovascular system and there is no antidote available for it. In spring 2006, three male German shepherd dogs (kept as sentinels in a factory located in a suburb of Tehran) were referred to the clinic for routine check up and anti parasite prophylaxis. Levamisole was administered as anthelminthic drug with inappropriate dose (80mg/kg). Two days later, all dogs were referred to the clinic with severe symptoms such as vomiting, bradycardia, tachypnea, excitement or depression. ECG abnormalities included ventricular fibrillation, and ventricular and atrial complexes. These disorders caused the death of dogs. Autopsy showed salivary discharges in trachea, and congestions in lung and intestine. Levamisole intoxication was finally recognized based on the clinical symptoms and pathology results.