The thesis begins with a description of recent advances in the development of microelectrodes and chemically modified electrodes and reviews some of the more important and interesting examples of their an analytical applications. The second chapter describes the development of a chemically modified electrode for the amperometric detection of nitrite. The sensor is based on the immobilisation of a ruthenium polymerin a carbon paste matrix which serves to lower the overpotential for nitritoxidation, hence permittingits determination at a lower potential. An investigation into the electrochemical pretreatment of carbon fibre microelectrodes is presented in chapter 3, and a pretreatment regime was optimised for the determination of folic acid. The effects of the pretreatment on the functional characteristics of the microelectrode is also presented. In chapter 4, the design and evaluation of two capillary electrophoresis systems, one employing a UV/VIS detector, and the other employing an amperometric detector based on a carbon fibre microelectrode is described. Finally, in chapter 5, and presented in two sections, is the application of microdialysis sampling coupled to capillary electrophoresis with electrochemical detection (employing the system developed in chapter 4), for in vivo studies. The first study involves a pharmacokinetic investigation of the drup L-dopa. The second study presented is a neurochemical analysis of amino acid neurotransmitters.