To understand how the brain functions and what disruptions underlie neurological diseases and disorders, analytical methods are needed that can succeed in the complexity of the native brain environment. To make a measurement in functioning, live tissue, these methods must be selective for specific analytes in a matrix that has over 1000 different chemical species, be able to measure chemical changes on multiple timescales (10-3 s to 104 s), have a high spatial resolution (μm), and be sensitive (pM to μM). The work described within, details the development and application of a voltammetric method, fast-scan controlled adsorption voltammetry (FSCAV) that is capable of monitoring baseline levels of serotonin and dopamine, as well as monitoring changes on multiple time scales with high sensitivity and selectivity. Because FSCAV is performed using a carbon-fiber microelectrode, the same sensor can be used for fast-scan cyclic voltammetry to monitor rapid (phasic) changes of dopamine and serotonin in the extracellular space. Thus a single-sensor strategy for measuring tonic and phasic concentrations of these important neurotransmitters is developed and used to elucidate important insight into the differences of serotonin and dopamine regulation. Additionally it is revealed that dopamine exhibits a coaction between tonic and phasic signaling where serotonin does not. Using this approach, a method for evaluating pain processing in a preclinical model is developed, which reveals an important relationship between chronic pain and dopamine signaling. Furthermore, a mathematical model to describe mass-transport limited adsorption is developed and used to determine the diffusion coefficient of both dopamine and serotonin in situ. The work described within details an important advancement in neuroanalytical methodology that will provide new insights both short-term and long-term for studying fundamental chemical mechanisms of neurotransmission.