A method for the simultaneous measurement of norepinephrine, epinephrine, dopamine, 3,4-dihydroxyphenylacetic acid, homovanillic acid, serotonin and 5-hydroxyindole-3-acetic acid in cat and human plasma by in vitro microdialysis-microbore high-performance liquid chromatography with electrochemical detection is described. The detection limit (signal-to-noise ratio = 3) is about 0.05-0.1 pg per injection. The volume of plasma samples required is very small (< 200 microliters), hence there is minimal blood loss in repeated blood sampling, especially in experiments using small animals. Within 15 min, a fast isocratic separation of these analytes by using a microbore reversed-phase ODS column is achieved, hence over 90 analyses can be performed in a single working day. As microdialysis per se is not destructive to plasma samples, the remaining plasma sample and perfusate can be repeatedly analysed for other substances. This simple, efficient and sensitive method can therefore be used as a routine clinical and basic research technique in the investigation of blood biogenic amines and their metabolites.