Changes in the serotonergic system in the posterior hypothalamus of freely moving rats were related to sleep and wakefulness using in vivo voltammetry (with carbon fiber microelectrodes) and polygraphic recordings. By using an optoelectronic telemetry system for the voltammetric signals, electrical cross-talk between the two settings was avoided and simultaneous neurochemical and electro-physiological recordings could be made so that a detailed time course of events could be obtained. Extracellular levels of the serotonin metabolite, 5-hydroxy-indoleacetic acid, measured every 2 min, increased with wakefulness and decreased with sleep: levels were significantly lower during desynchronized sleep than slow wave sleep. In vivo voltammetry associated with the optoelectronic telemetry system appears to be a useful tool for studying the relationship between neurochemical changes and electrophysiological events.