鈥� Excretion of 3-methoxy-4-hydroxyphenylglycol (MHPG) was measured repeatedly in 17 normal subjects. The amount of MHPG excreted over a 24-hour period was fairly stable over a period of three consecutive 24-hour samplings, but stability was rather poor over subsequent periods of several weeks, suggesting that execretory patterns are not traits. A normal range of MHPG excretion was estimated to be 900 to 3,500 碌g/24 hr. This range covers the majority of persons with affective disorders whose excretion patterns have been measured, although comparisons of absolute values between laboratories must be made cautiously. Further, substantial changes within this range may occur in normal subjects with no accompanying change in affect. A slight but definite diurnal pattern of excretion was found, with a peak at the period of 1600 to 1800 hours. No clear relationship to MHPG excretion to state of physical activity, recent consumption of foods or beverages, or prevailing affective state was defined in those subjects living under normal conditions. While MHPG excretion may yet prove useful for categorizing depressed patients and predicting response to drugs, any inferences drawn regarding the pathogenesis of affective disorders must be guarded.