Muscle vasoconstrictor (MVC) sympathetic preganglionic neurone (SPN) activity plays a key role in determining blood pressure. We have employed a whole cell recording (WCR) approach to functionally identify putative MVC SPN in the working heart-brainstem preparation (WHBP, Paton, 1996) that will allow an assessment of neuronal mechanisms governing their activity. Neonatal Wistar rats (p5-12, n=33) were anaesthetised with halothane, decerebrated precollicularly and perfused with carbogenated Ringer’s solution (32°C). After laminectomy (T1-T8), the spinal cord was transected at the level of T3 using a custom made vibroslicer. Phrenic nerve activity was recorded and the thoracic ventral roots stimulated with bipolar electrodes. Patch electrodes were positioned under direct visual control, in the lateral horn, to obtain WCR (pipette solution contained Lucifer yellow). Respiratory drive could be either increased by stimulating peripheral chemoreceptors (NaCN, i.a.) or arrested by topical cold saline to the snout to evoke the diving response. 58 SPNs were identified on the basis of their characteristic electrophysiology (Dembowsky, 1986, Pickering, 1991), and 17 of them were definitively identified by ventral root stimulation and a further 5 by morphology. In the T3 segment, SPN were either spontaneously firing (n=49) or quiescent (n=9). 34% of SPN (n=20) were identified as MVC on the basis of excitation during both peripheral chemoreflex and diving response activation. Compared to other SPN the MVC SPN had more hyperpolarised resting potentials (-53±1.4 vs. -48±1.4 mV; p=0.02) and lower input resistances (246±12 vs. 282±19 MOhm, p=0.04). The action potential parameters were similar across the SPN populations. We have observed a number of different patterns of respiratory modulation of MVC SPN activity (n=9/20). In 30% of MVC SPN (n=6) excitation occurred in late inspiration/early post-inspiration and inhibition during late expiration/early inspiration. This biphasic pattern was generated by bursts of inhibitory and excitatory post-synaptic potentials. In the remaining MVC SPN (n=11) the pattern of respiratory modulation was less obvious under basal conditions. However, increasing the respiratory drive by activation of the peripheral chemoreflex revealed a similar pattern of respiratory modulation with late expiratory inhibition and late inspiratory excitation (n=5/11, 46%). Using WCR in this in situ preparation, we have functionally identified MVC SPN and initiated the characterisation of the respiratory drives onto these neurones.