Practical utilization of in vivo electrochemistry for brain studies has been impeded by difficulties with specificity and quantitative measurements. Studies in brain slices combined with similar data from intact animals have provided important new information on the nature of the electrochemical studies. Using the enzyme ascorbic acid oxidase (AAO), the primary signal contribution has been calculated to come from ascorbic acid. By inhibiting monoamine oxidase activity with pargyline, the second most important contributor in caudate extracellular fluid is postulated to be 3,4-dihydroxyphenylacetic acid. These species are calculated to constitute over 93% of the observed oxidation current in rat caudate extracellular fluid.