Neurotransmitters are small molecules that are secreted from neurons into the extracellular space to communicate with adjacent neurons. Once released, the neurotransmitters diffuse to receptors, specific proteins on the surface of neurons that bind to the neurotransmitter. This binding causes an alteration in the properties of the neuron. Through this mechanism, information can be processed in the brain. It has been demonstrated that microelectrodes can be used to detect neurotransmitters as they diffuse through the extracellular space. This provides a unique way to measure these substances as they relay information and to probe the mechanisms that regulate their actions. Examples will be shown in which the neurotransmitter dopamine is detected on a subsecond time scale in the brain of a behaving animal with a carbon-fiber microelectrode.