Following the observation of Anderson that ingestion of bananas produces an increased excretion of 5-hydroxyindoleacetic acid in the urine, the present authors undertook a study to identify compounds in banana which could be the basis for this phenomenon. It was found that banana contained large amounts of norepinephrine and a related substance, dopamine. Large amounts of serotonin were also identified in banana. These are potent physiologic agents and thus their presence in a food is of considerable clinical interest. Whether the oral administration of these substances through banana feeding results in characteristic physiologic effects of these substances, and whether some of the reported therapeutic uses of banana can be attributed to their presence, remain to be determined.