The urinary excretion products of serotonin (5-hydroxytryptamine, 5HT) are 5-hydroxyindole-3-acetic acid (5HIAA) and 5-hydroxytryptophol (5HTOL), and the ratio of 5HTOL to 5HIAA is normally very low (< 0.01 ) in man. Intake of foods rich in 5HT (high amounts in banana, pineapple, and walnuts) induces a general increase in the output of 5HT metabolites, without affecting the 5HTOL/5HIAA ratio. In contrast, during metabolism of ethanol there is a shift in the catabolic pattern of 5HT, and the formation of 5HTOL increases appreciably at the expense of 5HIAA. Accordingly, the urinary 5HTOL/ 5HIAA ratio increases and does not recover to baseline levels until several hours after ethanol has been cleared from the body. When 10 healthy subjects ingested a moderate dose of ethanol (0.5 g/kg), the urinary 5HTOL/SHIAA ratio was increased approximately 70-fold on average at 4 h after intake. When the same amount of ethanol was ingested together with 3 bananas (approximately 10 mg 5HT), this ratio was increased approximately 100-fold at 4 h and still significantly higher than baseline levels at 24 h. Starting at 3-4 h after the combined intake of ethanol and banana, 7 subjects experienced one or more unpleasant symptoms (diarrhea, headache, and fatigue) which are associated with the 5HT system. The events were transient but typically lasted for several hours, and the duration correlated with the time period during which 5HTOL levels were raised. Intake of ethanol and banana separately produced much lower increases in 5HTOL output and caused no corresponding effects. This observation indicate that dietary 5HT intake together with even a moderate dose of ethanol can provoke unpleasant physiological symptoms. The symptoms may be attributed to the high concentration of 5HTOL.