Where two neurons meet they can form a synapse and chemical communication can occur. This is the functional unit of the brain, and it is at this level where changes equate to learning. The proposed research will develop and then use electrochemical methods to measure the absolute concentrations of easily oxidized neurotransmitters in vivo - something not possible with current methodology. Fast changes in the extracellular concentration of neurotransmitters can arise from phasic neuronal firing. For this reason, chemical sensors should be able to operate on a wide range of time scales. An ideal sensor for the detection of neurotransmitters has high sensitivity, can distinguish between compounds, and has a fast response time. Electrochemical approaches offer a way to accomplish this for easily oxidized neurotransmitters by using an electrode next to sites where the neurotransmitter is released. Background-subtracted cyclic voltammetry, one widely used method is capable of measuring changes in neurotransmitter concentrations on a sub-second timescale. This methodology has proved to be a valuable tool to measure the concentrations; however, it cannot measure the absolute concentrations of neurotransmitters. The absolute concentration is of critical importance as it affects the receptor occupancy and can change the effect of released neurotransmitters on the receptors. A novel method to measure the absolute concentrations of easily oxidizable neurotransmitters on a single-second timescale will be developed. This method relies on the adsorption of these molecules to the sensing surface. Background-subtracted cyclic voltammetry is typically performed in a kinetically limited regime. In this experiment, we allow the amount adsorbed to the surface to vary by changing detection parameters. By allowing the system to come close to equilibrium, and then pushing it farther away, the absolute concentration of neurotransmitter present can be measured. The method will be validated by using well-characterized drugs. The effects of cocaine and methylphenidate (Ritalin), a psychostimulant drug on the absolute dopamine concentration in the rat striatum will be determined. Methylphenidate is approved for the treatment of attention-deficit hyperactivity disorder, postural orthostatic tachycardia syndrome, and narcolepsy. Methylphenidate, like other stimulants, increases the concentration of dopamine in the brain; however how methylphenidate changes these concentrations has not been measured with exquisite temporal resolution.