The in vivo performance of a voltammetric microprobe based on overoxidized poly(1,2-phenylenediamine) coated carbon fiber microelectrode (OPPD/CFME), developed in our laboratory, is presented. For this purpose, an OPPD microprobe was stereotaxicaly implanted in the striatum of a deeply anesthetized Wistar rat for the simultaneous measurement of dopamine, serotonin and ascorbate. Furthermore, the post mortem levels of these physiologically important compounds were monitored after the rats were terminated with an overdose of anesthetic introduced through an indwelling jugular catheter. Using cyclic (CV) and square-wave (SWV) voltammetry, the OPPD/CFME was demonstrated to exhibit efficient separation of the voltammetric signals of dopamine, serotonin and ascorbate in the presence of biological matrix, with the SWV mode allowing more convenient detection regarding both sensitivity and selectivity. Explicit proof of in vivo dopamine detection at the OPPD/CFME was achieved via the absence of the dopamine signal in rats with unilateral lesions of nigrostriatal dopaminergic neurons, which was induced by the use of the selective dopaminergic neurotoxin 6-hydroxydopamine (6-OHDA). In stark contrast to non-treated rats, where a strong signal corresponding to 1.2 micro M dopamine was measured at ca. 90 s after the rats' death, the signal for dopamine in dopamine-depleted striatum of 6-OHDA rats was absent. A critical comparison of the in vivo performance of the OPPD/CFME to that of bare and Nafion-coated carbon fiber microelectrodes, often used in neurophysiological studies, clearly showed a significant advantage of the former microprobe. The OPPD/CFME allowed multiple and repetitive in vivo measurements, along with pre- and post-measurement external calibration, with no loss in selectivity and an acceptable loss in sensitivity, indicating that the active sensing sites were not adversely blocked by the components of the extracellular fluid, thus affirming the great practical in vivo applicability of the OPPD microprobe.