Plasma catecholamine levels and the urinary excretion of catecholamines and catecholamine metabolites were studied in two male volunteers during a cycle of morphine addiction and withdrawal. Plasma catecholamine values Were within normal limits except for an elevated epinephrine level in Subject 1 on the first day of withdrawal. At the beginning of the addiction phase the excretion of epinephrine, norepinephrine, dopamine, 3-methoxy-4-hydroxymandelic acid, and the catecholic acid fraction, consisting predominantly of 3,4-dihydroxyphenylacetic acid, rose significantly in both subjects. The excretion of metanephrine and normetanephrine was also increased in Subject 1, but not in Subject 2. The excretion rates, with the exception of the catecholic acid fraction in Subject 2, returned to normal or below normal during the withdrawal phase. These results indicate a stimulation of catecholamine discharge during the addiction phase but provide no evidence for its stimulation during the period of withdrawal.