Influence of labetalol and 5-(1-hydroxy-2-aminoethyl)salicylamide (SA), a part of the chemical structure of labetalol, on the fluorimetric assay of catecholamine (CA) was studied. Both labetalol and SA have a weak but significant fluorescence which is indistinguishable from that of CA with peaks of excitation/emission wavelengths at 410/490. It is thus concluded that an apparent increase in urinary CA observed in patients receiving labetalol is caused by the contamination of labetalol and/or its metabolite, and that the evaluation of urinary CA in hypertensive patients must be done prior to the use of labetalol to avoid any confusion in diagnosis of pheochromocytoma.