The efficacy of cimetidine administration after the open heart surgery was studied using the cimetidine concentration examination in the twenty five consecutive cases. Cimetidine was administered intravenously. The cases were divided into five groups for the infusion period. There was no case which developed the gastro-duodenal bleeding after the surgery. The concentration of cimetidine has showed more than 0.5 microgram/ml in each group. It was concluded that the administration of 800 mg cimetidine infused more than three hours, repeated every twelve hours was the possible treatment for the prevention of the gastro-duodenal bleeding after the open heart surgery.