A simple, rapid and highly sensitive method for the determination of catecholamines (norepinephrine, epinephrine and dopamine) in human plasma is described which employs high-performance liquid chromatography with fluorescence detection. After cation-exchange chromatography on a Toyopak SP cartridge, the catecholamines and isoproterenol (internal standard) in 500 microliters of plasma are converted into the corresponding fluorescent compounds by reaction with 1,2-diphenylethylenediamine in aqueous acetonitrile. These compounds are separated within 8 min on a reversed-phase column, TSK-gel ODS-120T, with isocratic elution using a mixture of water, methanol and acetonitrile containing a Tris--hydrochloric acid buffer (pH 7.0). The detection limit for each catecholamine is ca. 2 fmol in a 100-microliters injection volume. N-Methyldopamine can also be used as internal standard.