Carbon-fiber microelectrodes can serve as chemical sensors for the detection of easily oxidized chemical messengers such as dopamine, serotonin, and histamine in biological systems. The electrodes have micron dimensions and can be used on millisecond time scales. Thus, they can be used to measure neurotransmitter release at the level of single cells or in the brain of intact, behaving animals. Such measurements are giving new insights into the complex chemical interactions that regulate behavioral states.