In vivo differential pulse voltammetry was used to determine the effect of thyrotropin-releasing hormone (TRH) on dopaminergic and serotoninergic synaptic activity in the striatum and nucleus accumbens of the rat. Thyrotropin releasing hormone (TRH) produces marked stimulatory effects on behaviour, which have been attributed to the release of dopamine in the nucleus accumbens. Other studies indicate a close relationship between the peptide and serotonin in the brain. We have thus used an improved differential pulse voltammetry technique to evaluate the effects of TRH on the extracellular content of the dopamine and serotonin metabolites. Dihydroxyphenylacetic acid (DOPAC) and 5-Hydroxy-indoleacetic acid (5HIAA) in the nucleus accumbens and striatum of the rat in vivo. TRH rapidly increased extracellular DOPAC, reaching a maximum after 60 min in the nucleus accumbens, and after 40 min in the striatum. There was also a slower increase in extracellular 5-HIAA content in both areas, reaching a plateau after 100 min. The delayed time course of the increase in 5-HIAA suggested that the increase in 5-HIAA content might be secondary to the increase in dopamine turnover produced by TRH. These results suggest that doses of TRH which produce behavioural stimulation increase the release of both dopamine and serotonin in the nucleus accumbens and striatum.