The aim of this thesis is to further demonstrate the electrochemical detection of nitric oxide (NO) and hydrogen peroxide (H2O2) in-vitro, to advance the previously demonstrated detection of brain NO and to demonstrate the novel in-vivo detection of H2O2 using a paired catalase-based biosensor. We have recently successfully demonstrated the real-time detection of brain NO using a previously characterised Nafion®-modified platinum (Pt) electrochemical sensor. Additionally, the preliminary in-vitro development of a paired catalase based Pt sensor for the purpose of H2O2 detection in the physiological environment has previously been reported by our research group. A validation of the previously reported in-vitro characteristics of the NO sensor with the inclusion of novel post in-vivo stability attributes is presented in this thesis (see Chapter 6). Also, the suitable functionality of the NO sensor in the neuronal environment as has been published recently is supported in Chapter 6. A verification of the previously reported in-vitro attributes of the paired catalase based H2O2 sensor is shown in Chapter 4. Additionally, advancements to the initial in-vitro characterisation of the paired H2O2 sensor are described in Chapter 4. The previously unreported real-time detection of brain H2O2 in freely-moving animals is presented in Chapter 5. The main focus of this thesis is the detection of NO and H2O2 as oxidative/nitrosative stress markers in neurological disease and dysfunction. As such the role of oxidative/nitrosative stress in Parkinson’s disease (PD) and autism spectrum disorders (ASDs) is investigated in this thesis. The novel detection of NO in a propionic acid (PPA) induced animal model of ASDs is presented in Chapter 7. The electrochemical detection of NO and the preliminary dual detection of NO and H2O2 in a paraquat (PQ) mediated model of PD is presented in Chapter 8. The specific focus of these investigations is to elucidate the role of oxidative/nitrosative stress in the etiology and pathophysiology of PD and ASDs.