We present a case of malignant arrhythmias associated with hypocalcemia and subsequent cardiac arrest in a 20-year-old woman due to ingestion of 3.1 g mesoridazine in a suicide attempt. There were unexplained alterations in serum electrolytes preceding the cardiac arrest. The patient subsequently recovered and was transferred to a psychiatric hospital. Despite the paucity of data on the potential cardiotoxicity associated with mesoridazine overdose, this case demonstrates that malignant arrhythmias and cardiac arrest occur; consequently, these patients probably require constant ECG monitoring.