The radioenzymatic determination of plasma catecholamines with a modification of the method of da Prada & Zürcher ((1976), Life Sci 19, 1161-1174) is described. The several reaction steps were optimized with respect to the quantities of substrate and enzyme, and reaction time. There were particular methodological difficulties concerning the blanks, which were determined by using sodium metaperiodate-oxidized plasma. The reliability criteria of the method were determined. Coefficients of variation between 3.4 and 8.6% were found for the intra-assay variability of 10 pg of norepinephrine and 3 pg of epinephrine or dopamine, resp. The recoveries o the three catecholamines ranged from 88-93%. The detection limits were calculated from the standard deviation of the blanks and amounted to 12 ng/l (norepinephrine), 6 ng/l (dopamine) and 3 ng/l (epinephrine). The method was used for the analysis of plasma samples from patients. In a further investigation we examined the stability of plasma catecholamines stored at different temperatures. It was found that samples can be stored for 1-2 hours at room temperature and for several weeks at -27 degrees C without losses in catecholamine content.