A fluorimetric assay for the determination of 5-methoxytryptamine and serotonin in blood and urine is discussed, which is based on the reaction with 3-ketobutyraldehyde-1-dimethylacetal and hydrogen peroxide. Only 5-methoxytryptamine enters the analysis cycle by extraction with benzene and is determined selectively. By contrast, both amines are measured quantitatively with ethyl acetate extraction, so that the difference between the two analytical results determined using different extractants corresponds to the present serotonin content. The procedure discussed was used to detect 5-methoxytryptamine in the urine (36.55 ±5.23 microg/24 hours). In normal human blood, 5-methoxytryptamine was found only in 4 out of 50 persons examined, in the order of 0.038 microg/mL.