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Lithium is the most frequently used mood stabilizing drug in the bipolar affective disorder treatment and prophylaxis. Its therapeutic index is extremely narrow. In addition to its neurological, gastrointestinal, renal, and endocrine side effects, it can rarely cause the cardiovascular side effects such as the life threatening cardiac arrhythmia. The cardiac symptoms can generally be determined in EKG. The changes such as T wave şattening in EKG due to the lithium usage, slowing in the ventricular conduction, elongation in the QT interval and atrioventricular block can be observed. In this presentation, the male patient in the age of 42, whose sinus bradycardia has developed with the lithium treatment will be presented. 42 year-old male patient has been monitored with the diagnosis of the bipolar affective disorder for 18 years. His attacks have been under control for 8 years with the lithium 900 mg/day treatment and he has been in full remission. Two weeks after the lithium dose was increased to 1200 mg/day, he applied to the emergency service with the complaint of the fainting. In the first evaluation of the patient, it was determined that his pulse was 30, and his tension was 60/40. In his cardiovascular examination, the findings other than the bradycardia could not be recorded. In the ECG, the slow ventricular response atrial fibrillation was observed. In the coronary angiography no pathology was found. After the temporary transvenous pacemaker was inserted to the patient, and enoxaparine was started for atrial fibrillation. Upon the correspondence of the lithium value to the 1.8 mEq/dl, the lithium was stopped and the liquid treatment was continued. With the decline of the lithium level to 0.4 mEq/dl on the 5th day of the treatment, the cardiac rhythm turned to the sinus rhythm. The patient was discharged by passing to another mood stabilizing valproic acid. The cardiac symptoms of the lithium occur depending on the place changed of the intracellular potassium with the lithium and they are similar to the ECG changes that the hypokalemia has formed: Flattening in the T wave, elongation in the QT interval and AV conduction delay and bradycardia can be observed. Due to the fact that the lithium represses the pacemaker in the sinus node, the life threatening bradycardia, hypotension, syncope and general condition impair can be observed. The long lasting lithium treatment shows cardiac toxic effect and can cause arrhythmia forming dangers to the life. If the patients, who apply to the emergency service have lithium usage story, it is required to research the lithium level by considering that the arrhythmia could be developed.