Near infrared spectroscopy (NIRS) was first used as a tool for the in vivo monitoring of tissue oxygenation in the late seventies. Today, NIRS instruments are more and more used in clinical environments since they are now easy to use, sensitive, robust, provide rapid analysis and could be complementary to other non invasive methodologies such as functional magnetic resonance imaging (fMRI) and positron emission tomography (PET). The feasibility of non-invasive analysis of brain activities is studied in the attempt to overcome the major limitation of invasive in vivo methodologies. In the present work, optic fibre probes were used as optical head of a novel, highly sensitive near infrared continuous wave spectroscopy (CW-NIR) instrument adapted for in vivo NIRS measurements in the brain of rodents. This prototype was designed for non-invasive analysis of the 2 main forms of haemoglobin: oxy-haemoglobin (HbO2) and deoxy-haemoglobin (Hb), chromophores present in biological tissues as these are markers of the degree of tissue oxygenation, thus providing an index of blood level and therefore of tissue metabolism. It was first tested in peripheral tissue (human gastrocnemius muscle) and then reset to perform measurement on rat brain. In animal studies, the optical head was firmly placed using stereotaxic apparatus upon the sagittal line of anaesthetised adult rat ’ s head, without any surgery. ‘Physiologic’ (i.e. with exogenous oxygen (O2) or carbon dioxide (CO2) supplied orally) or pharmacologic (i.e. with drugs of abuse such as cocaine) experiments have been performed to support the effectiveness of the methodology in preclinical studies. In addition, the possibility that changes in brain metabolism as measured by NIRS might be a useful index of brain penetration of new chemical entities has been investigated using different compounds from different chemical classes that were selected on the basis of their known brain penetration and overall pharmacokinetic profile. Finally, the feasibility of coupling NIRS with another although invasive in vivo method such as electrophysiology for concomitant analysis of cerebral cell firing in discrete brain areas was tested in the attempt to study in real time the putative correlation between blood levels, brain metabolism and neuronal activities in rat CNS, i.e. apply NIRS to pharmacodynamic investigations. The data gathered in rat treated with exogenous O2, indicate an original relationship between NIRS analysis of brain metabolism and electrical changes in this major nucleus of CNS involved in neurophysiologic and pathologic activities.