The possibility of using in vivo voltammetry to monitor 5-hydroxytryptamine (5-HT) release from brain tissue in freely moving unanaesthetized rats has been examined. A potential (+0.2 to +1.0 V) was applied to a micrographite electrode stereotaxically placed within a specific brain region and current changes following the oxidation of electroactive compounds in the vicinity of the electrode tip were recorded. Administration of p-chloroamphetamine (5 mg/kg) produced a large increase in current in the striatum and this could be prevented by pretreatment with p-chlorophenylalanine (150 mg/kg X 2) to deplete brain 5-HT or Fluoxetine (10 mg/kg) which prevents the uptake of p-chloroamphetamine by 5-HT neurones. Fluoxetine (10 mg/kg) caused a small but long lasting increase in current. Stimulation of the median raphe nucleus produced a marked and rapid rise in current in the hippocampus but a much smaller one in the striatum. This response could also be prevented by 24 h pretreatment with p-chlorophenylalanine (150 mg/kg). Seven days after p-chlorophenylalanine administration raphe stimulation again produced an increase in current. Rats under barbiturate anaesthesia showed no clear increase in current either after p-chloroamphetamine or raphe stimulation, indicating that barbiturates may affect neurotransmitter release. The results suggest that 5-HT release can be monitored in the freely moving unanaesthetized rat using in vivo voltammetry, and that a moderate decrease in brain 5-HT concentration leads to a substantial inhibition of drug or stimulation induced release of 5-HT.