The diurnal variation of urinary vanillylmandelic acid (VMA) and metanephrine-normetanephrine levels was studied in six patients with phechromocytoma and 32 patients with essential hypertension. Despite variable degrees of day-night differences, urinary excretion of VMA and metanephrines, per unit of time or per gram of urinary creatinine, was invariably higher in patients with pheochromocytoma than in controls whether the collection was 7 AM through 7 PM or 7 PM through 7 AM. Thus, shorter study periods are sufficient to exclude the presence of a chromaffinoma in most patients with hypertension.