Intravenous administration of tritium-labeled 3,4-dihydroxyphenylalanine (dopa) to human subjects resulted in the labeling of endogenous catecholamines and vanillylmandelic acid (VMA). Determination of the changes in specific activity of these compounds with time in fractional collections of urine and in cardiac biopsies from patients undergoing corrective cardiac surgery permitted estimation of apparent turnover rates. The average half-time of the exponential decline in specific activity of labeled urinary norepinephrine was about 8 hr and that of VMA was 11-16 hr in five normal subjects. No significant differences from normal were observed in eight patients with essential hypertension. The average half-life of norepinephrine was only 5 hr in cardiac patients undergoing surgery, and the levels and rate of decline of cardiac norepinephrine specific activity correlated well with the exponential phase of the urinary disappearance curve. There were significant effects of treatment with alpha-methyltyrosine, reserpine, and pargyline hydrochloride on the labeling and apparent turnover rates of norepinephrine and VMA, the effects noted were consistent with known actions of these three drugs. It is suggested that the technique used is a suitable means of assessing over-all catecholamine metabolism in man, particulary if combined with quantitative assay of urinary catecholamine metabolites.