The NTS, a major site for blood pressure (BP) control, contains neurons receiving baroreceptor input. One neurotransmitter affecting cardiovascular control is serotonin (5-HT), which can modify NTS neuronal activity by several different 5-HT receptor subtypes (Sévoz-Couche et al. 2000). Here, we investigated whether barosensitive NTS neurons are directly innervated by serotonergic axons.In conscious rats expression of Fos was evoked by BP increases (125% baseline) due to 90 min i.v. phenylephrine infusions (Minson et al. 1997). After 2 hr the rats were anaesthetised (pentobarbitone, 100 mg kg^-1 i.p) and perfused with fixative. Medullary sections were immunostained with peroxidase to reveal Fos and either 5-HT or serotonin transporter (SERT) and processed for light (LM; n=7 rats) or electron microscopy (EM; n=4). In another set of pentobarbitone anaesthetised (60 mg kg¹ i.p.), neuromuscularly blocked rats (Jones et al., 2002), NTS neurons responding to vagus (n=4 cells) or aortic nerve (AN; n=4) stimulation were juxtacellularly labelled with neurobiotin (Pinault, 1996). Filled neurons were detected with avidin-peroxidase and 5-HT, with immunohistochemistry in sections of fixed medulla.Raising BP induced strong Fos expression in many neurons in the baroreceptor region of caudal NTS. LM showed Fos-immunoreactive (IR) nuclei within a network of varicose 5-HT-IR or SERT-IR axons, suggesting that 5-HT terminals might appose barosensitive NTS neurons. EM showed that 5-HT-IR boutons only rarely formed asymmetric synapses on unidentified dendrites in the baroreceptor region. Thin glial processes were usually interposed between serotonergic terminals and NTS cell bodies, but in one case, a 5HT-IR terminal directly apposed a Fos-IR cell body without forming a morphologically-identifiable synapse. Intense SERT-IR occurred in intervaricose axons but terminals stained only lightly.Juxtacellular labelling revealed cell bodies and dendrites of neurons responding to vagus or AN stimulation and these received occasional or no close appositions from 5-HT-IR axons. The apposition density was so low that EM was not worthwhile.Our data indicate that, if 5-HT axons synapse on barosensitive NTS neurons, innervation is sparse and occurs mainly on dendrites. Our EM evidence suggests that glial cells may be involved in serotonergic transmission within the NTS.