The NTS is richly innervated by 5-HT terminals (Steinbusch, 1981) originating centrally (Schaffar et al. 1988) and from vagal afferents (Nosjean et al. 1990). Vagal afferent activation of NTS neurons has been shown to be mediated by 5-HT₃ and 5-HT₇ receptors (Ramage & Villalón, 2008). This indicates vagal afferents cause the release of 5-HT within this nucleus. Experiments were carried out to determine if this release could be detected in real time in anaesthetized rats using fast cyclic voltammetry. Sprague-Dawley rats (250-330g) were anaesthetized with isofluorane (5% in 100% oxygen) and maintained with either sodium pentobarbitone (60 mg kg^-1, i.v.) or α-chloralose (120 mg kg^-1, i.v.), neuromuscular blocked (α-bungarotoxin 150 μg kg^-1, i.v.) and artificially ventilated with O₂-enriched air. Depth of anaesthesia was assessed by the stability of BP and HR following a noxious stimulus and additional anaesthetic was given when necessary. NTS was exposed by removing the occipital bone. The left vagus nerve was exposed and tied distally to the stimulating site. A carbon fibre electrode (tip dia. 7-10 µm) was then inserted into the medial NTS and lowered until evoked potentials could be detected from vagal stimulation (10-400 μA, 0.2-1.0 ms, 10x threshold). The sites were confirmed histologically. The system was then switched to voltammetry. A modified form of fast differential scan voltammetry (Millar & Williams 1990) using trapezoidal (flat-top) scans limited to a +450 mV positive potential for the electrochemical detection of 5-HT was used. This waveform is highly selective, detecting 5-HT at levels of <10 nM. For dopamine or noradrenaline and ascorbate the detection thresholds are >100 nM and >1 µM, respectively. All values are means ± S.E.M. Responses were measured as the height of oxidation peaks. Under barbiturates anaesthesia vagal stimulation (20 Hz) evoked an increase by 6±1 nM (n=5), a decrease by 11±4 nM (n=5) and one biphasic effect change in 5-HT levels, while under chloralose increases were mainly observed of 23±5 nM (n=14). In some cases there was no response at 20Hz, however at 50Hz an increase of 22±8 nM (n=7) was observed, while in others release could be observed at 5 and 10Hz of 29±9 nM (n=3) and 28±6 nM (n=5). Citalopram (100 µg kg^-1; i.v.; 20Hz) tended to cause a decrease in release from 20±7 to 15±5 nM. Increasing the dose had no further effect. Cadium applied topically to the NTS (10^-3 M in 100µl; n=1) completely abolished release after 1st stimulus in a train of three. In one experiment using chloralose a decrease in 5-HT was observed. The data indicates that vagal afferent stimulation can produce increases in the level of extracellular 5-HT and this seems to be an all or nothing response.