A method for the measurement of norepinephrine (NE) overflow from isolated superfused canine saphenous veins is described. This method involves concentrating NE present in the superfusate, followed by measurement of its concentration by high-performance liquid chromatography with electrochemical detection. The limit of sensitivity of the method (defined as a signal-to-noise ratio of 5) was 25 pg. NE concentration measured by this method correlated well (r = 0.95) with that measured by standard fluorometric methods. Electrical stimulation caused an initial overflow of a mean of 115 X 10(-18) mol NE/mg of vein per pulse; this was associated with an increase in isometric tension. With continued stimulation, less NE overflowed into the superfusate, although tension was not reduced appreciably.