In cases of severe malaria chloroquine phosphate is frequently given--diluted or undiluted--by the intravenous route. It is known that cardiac arrhythmias and hypotension may complicate such therapy, but cardiac arrest is not a well recognised hazard. In this report we describe such a tragic complication, and advocate that undiluted chloroquine should not be administered intravenously in severely ill patients since such patients usually have associated electrolyte disturbances which may render the heart vulnerable to toxic drugs. Chloroquine may be given diluted in normal saline infused over several hours with a close watch over the blood pressure.