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Selective and simultaneous voltammetric analysis of catechols and indoles in vivo and in vitro has until now been feasible only by means of 'slow' scanning methods (scan speed in tens of seconds) such as differential pulse (DPV) and differential normal pulse voltammetry in conjunction with electrically and/or chemically treated carbon-fiber micro-electrodes (mCFE). Faster electrochemical techniques, such as chronoamperometry and cyclic voltammetry (CV), allow more rapid (seconds or fractions of a second) and frequent measurements of these chemicals. However, these methods show poor sensitivity and selectivity in the presence of different electroactive compounds with similar oxidation potentials. In order to analyze whether the lack of sensitivity and selectivity of the fast voltammetric methods results from the rapidity of the measurement or from the use of untreated sensors, the methods of CV (scan speed: 1000 mV/s) and DPV (scan speed: 10 mV/s) have been applied with either untreated or electrically treated mCFE to analyze the in vitro oxidation potential and current values of DA and 5-HT. When associated with untreated mCFE, neither method was able to separate and selectively detect the two compounds dissolved together in an inert vehicle; the voltammogram recorded resulted in a single broad oxidation signal. In contrast, when these techniques were performed with electrically treated mCFE, oxidation signals for DA (peak A) and 5-HT (peak B) were monitored simultaneously at approximately + 65 mV and + 240 mV, with DPV respectively, and at + 120 mV and + 300 mV with CV, respectively. Additionally, CV with treated mCFE on anesthetized rats, simultaneously monitored two striatal signals at approximately + 100 mV and + 300 mV. The oxidation values (Em) and current levels (nA) of these peaks remained stable during control recordings. The current levels were selectively increased by peripheral injection of fluphenazine (DA antagonist) or of 5-hydroxytryptophan (precursor of serotonin). The chemical nature of these two peaks may therefore be considered catecholaminergic and indolaminergic, respectively. Hence, this report provides the first evidence for the feasibility of concomitant in vitro analysis of DA and 5-HT using a rapid scanning method such as CV. In addition, the values of current level (nA) obtained with CV-mCFE for DA and 5-HT are comparable to those monitored with DPV-mCFE, supporting the view that treatment of the sensor is a key point for increasing the selectivity and the sensitivity of these voltammetric techniques. The feasibility of using CV with electrically treated mCFE for fast in vivo analysis of catechol and indole activities is also demonstrated.