Abstract The effect of controlled exercise on plasma and urinary 3-methoxy-4-hydroxyphenyleglycol (MHPG) and vanillylmandelic acid (VMA) was investigated in normal subjects. After 3 days of bed rest, six normal volunteers engaged in controlled exercise adjusted to a constant 40% of the individual's maximum oxygen intake. The exercise program consisted of 4 miles of walking and a half-hour each of bicycle ergometry and treadmill walking. Both plasma VMA and MHPG were higher on the exercise day than on the resting day, while urinary VMA and MHPG levels did not differ from rest to exercise. The mean urinary MHPG/VMA ratio (0.46) differed markedly from the mean plasma MHPG/VMA ratio (2.3). Plasma and urinary MHPG apparently represent different indices of norepinephrine metabolism.