A major improvement in the selectivity of small graphite electrodes used for in vivo electrochemistry is described. The electrodes are coated with Nafion, a perfluorosulfonated polymer. This coating is practically impermeable to ascorbic acid and anionic biogenic amine metabolites and only slightly responsive to neutral metabolites. Thus it becomes selective for the cationic primary neurotransmitters, dopamine, norepinephrine and 5-hydroxytryptamine. Responses of Nafion-coated and untreated electrodes in vivo are compared.