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A modified glassy carbon electrode is used to examine the mass transport characteristics of the neurotransmitter dopamine (DA) in sodium montmorillonite films. The apparent diffusion coefficient (Dapp) of DA in the film is measured by chronocoulometry and cyclic voltammetry to be 2.3×10–9and 5.1×10–9cm2/s. According to the Nicholson theory, the standard rate constant (ks) of heterogeneous electron transfer reaction for DA is estimated to be 2.7×10–6cm/s by cyclic voltammetry. Montmorillonite has been modified on carbon fiber electrodes, and voltammetric response of some neurotransmitters and their metabolites has been examined. The variation of DA concentration in a rat brain with decreasing blood supply has been detected when the modified microelectrode is used in in vivo experiments.