Urinary MHPG was assayed by gas chromatography using a flame ionisation detector. The technic proposed was reproducible, repeatable and linear. The frequent values determined in adult subjects of both sexes were 8.64 1.34 mumol/24 h without any difference between the sexes. On the other hand, urinary excretion of total MHPG showed a positive correlation with age. It was excreted in the conjugated form, of which 50 p. cent was sulphoconjugated, 41 p. cent glucuroconjugated, and 9 p. cent free. Owing to its simplicity, this method may be applied to the study of mood disorders.