The main aims of this thesis were to develop and characterise monomer modified oxygen (O2) sensors that could withstand the harsh setting of an in-vivo environment, without the risk of alteration of the sensor components and the characterisation of nitric oxide (NO) sensors in-vitro, followed by the complete in-vivo characterisation under physiological conditions. The main result sought after for development of these novel O2 sensors, was the potential utilisation in a clinical setting. Chapter 1 introduces the background behind the monitoring of NO and O2 in the brain, the biosynthesis of NO in the brain, the biosynthesis of NO through the N-methyl-D-aspartate receptor (NMDA) and the potential role of NO in neurovascular coupling. Chapter 2 discusses the theory behind the project and Chapter 3 describes the experimental protocols utilised throughout this research. Chapter 4 discusses the development and characterisation of the monomer modified O2 sensors. The subsequent characterisation of the novel O2 sensors in-vivo is discussed in Chapter 5. The use of various perturbations to alter the O2 concentration for characterisation of the O2 sensors is described. The characterisation of the NO sensor types (Type 2 and Type 3 NO sensor) is discussed in Chapter 6, with response times, sensitivity and selectivity discussed in detail. The subsequent characterisation of these sensors in-vivo is described in Chapter 7. Systemic and local administrations of various inducers and inhibitors of nitric oxide synthesis were carried out to determine the sensors viability in monitoring the analyte in a physiological environment. In Chapter 8, another aspect of this research involving the simultaneous recording of the NO and O2 analytes in-vivo is described, which utilises a combination of the styrene modified carbon paste electrodes (SMCPEs) and methyl methacrylate modified carbon paste electrodes (MMCPEs) for the detection of O2 and the Type 2 and Type 3 sensors for the detection of NO. These results yield valuable information about the possible relationship between the two molecules. The final chapter (Chapter 9) details a summary of all of the chapters and concludes with a final discussion on the results attained.