Fast cyclic voltammetry at a carbon fibre microelectrode was used to measure 5-HT signals following electrical or chemical stimulation in rat substantia nigra pars reticulata slices. Chemical stimulation with (+)-amphetamine or veratrine gave signals which were indistinguishable from those of exogenous 5-HT. Electrical stimulation of sufficient duration gave voltammetric signals which were characteristic of 5-HT. Release of dopamine was not detected following either chemical or electrical stimulation. The 5-HT signals were attenuated by TTX and enhanced by fluvoxamine. It was not possible to demonstrate regulation of 5-HT release in the SNr by 5-HT1B autoreceptors using CGS 12066A or methiothepin. Signal following electrical stimulation were not enhanced by either benztropine or GBR12909, or modified in the presence of either quinpirole or sulpiride. We conclude that 5-HT release can be detected voltammetrically in the SNr; 5-HT release is likely to be from axon terminals, but somatodendritic DA release could not be detected.