1. The 24 h urinary excretion of free 4-hydroxy-3-methoxyphenylethylene glycol (HMPG), HMPG conjugated as glucuronide and HMPG conjugated as sulphate was determined in nine healthy volunteer subjects and six patients with phaeochromocytoma. In both groups of subjects most (97%) HMPG was in the conjugated form. 2. Although patients with phaeochromocytoma excreted five- to ten-fold the amounts of each of the forms of HMPG excreted by the control subjects the ratio of these different forms of HMPG to each other did not differ significantly between the groups. 3. In cerbrospinal fluid and brain about 80% of HMPG was free and of the conjugated HMPG in these two tissues most was in the glucuronide form. 4. In is concluded from these data that both HMPG sulphate and HMPG glucuronide have a substantial peripheral origin and that measurements of their urinary excretion cannot be used as an index of brain catecholamine turnover.