1. 1. The urinary excretion of catecholamines (nor-epinephrine and epinephrine) was studied in sixteen cases of pheochromocytoma, ninety-one cases of essential hypertensive vascular disease, thirteen normotensive healthy subjects, ten cases of Addison's disease and fourteen cases of essential hypertension following thoracolumbar sympathectomy. 2. 2. The source of nor-epinephrine in human urine is the sympathetic nervous system. Only a small fraction may be derived from the adrenal medulla. 3. 3. Excessive urinary excretion of epinephrine and/or nor-epinephrine in pheochromocytoma could be demonstrated in cases with persistent hypertension as well as in cases of paroxysmal hypertension during or between attacks. 4. 4. The methods used consisted in part of adsorption of the urinary catecholamines on precipitated aluminum hydroxide, followed by elution, desalting and concentration in vacuo. These extracts were studied by bio-assay, paper chromatography, photofluorometric evaluation and absolute quantitation of nor-epinephrine and epinephrine by chemical methods. 5. 5. Photofluorometric evaluation of urine extracts seems to constitute the most reliable test for pheochromocytoma. 6. 6. A short procedure, consisting of adsorption of catecholamines in hydrolyzed urine on an aluminum oxide column, followed by photofluorometric evaluation of the eluate, can be used as a rough screening test. When positive results are obtained, the longer and more specific method has to be applied for confirmation.