The distribution of free and conjugated forms of the serotonin (5-hydroxytryptamine) metabolite 5-hydroxytryptophol (5HTOL) in human urine was determined. 5HTOL was analyzed using a sensitive and specific gas chromatographic-mass spectrometric method. The sulfate and glucuronide conjugated forms were measured indirectly following enzymatic hydrolysis. Total 5HTOL levels in control samples ranged between 98-301 nM, in samples collected following ingestion of bananas, a food rich in serotonin, between 450-3292 nM, following alcohol consumption between 863-13326 nM, and in samples obtained from patients with serotonin producing carcinoid tumors between 1695-3793 nM. Free 5HTOL accounted for less than 4% of total 5HTOL in all samples. Sulfate conjugated 5HTOL was calculated to comprise about 17% of total 5HTOL in the control samples and 15% in the alcohol samples, whereas the mean proportion was significantly increased to 33% and 27% in the samples collected after ingestion of bananas and from patients with carcinoid tumors, respectively. The results show that conjugation with glucuronic acid followed by urinary excretion is normally the predominant route for elimination of 5HTOL in man. However, in situations of elevated levels of total 5-hydroxyindoles originating from dietary sources or serotonin producing tumors in the gut, sulfate conjugation becomes more important.