Urinary excretion of vanillymandelic acid (VMA) during a period of 24 hours was determined in 127 normal Korean adults and in 27 patients suffering from primary hypertension. The diurnal and nocturnal variations of urinary VMA excretion were measured in 3O normal persons and 11 patients with primary hypertension, and the day to day variations of urinary VMA excretion in l2 normal persons. The mean daily output of urinary VMA was fairly constant in each individual but varied widely between individuals. The mean daily output of urinary VMA in normal Korean adults is 1.95±1.15 (S.D.)mg, which is similar to that observed in occidentals. There is no significant difference between the nocturnal and the diurnal excretion of VMA. The mean daily output of urinary VMA in patients with primary hypertension was 2.17±0.76(S.D.) mg. This means that there is no significant variation in the urinary excretion of VMA between normal adults and patients with primary hypertension. Furthermore, the urinary output of VMA is not influenced by the sex.