Sympathetic outflows to particular target organs show specific patterns of respiratory modulation (Habler et al. 1994). Single unit recordings show striking differences in respiratory drive between sympathetic preganglionic neurones (SPN) (Gilbey et al. 1986). The central mechanism(s) underlying the differences in sympathetic respiratory modulation are not known. We addressed this question at the level of the SPN using whole cell recording (WCR). This study used the working heart brainstem preparation (Paton, 1996). Neonatal Wistar rats (p8-12) were anaesthetised with halothane, decerebrated precollicularly and perfused with carbogenated Ringer solution (32°C). Phrenic nerve activity was recorded and the thoracic sympathetic chain stimulated with a bipolar electrode. After laminectomy (T5-C8), the SPN were located using extracellular recording. Patch electrodes were driven into this area to obtain WCR (solution containing Lucifer yellow). Respiratory drive could be either increased by stimulating peripheral chemoreceptors (NaCN, 10-20μg, i.a.) or arrested by topical cold saline (10°C) to the snout to evoke a diving reflex. WCR were made from 62 spinal neurones (T2-T3, 500-900μm deep to dorsal surface). SPN were identified definitively by either antidromic activation or anatomical reconstruction. Neurones were identified as putative SPN on the basis of their distinctive electrophysiological properties (Dembowsky et al. 1986; Pickering et al. 1991). Of the 62 spinal cells 22 were identified as SPN (3 identified definitively and 19 classed as putative). SPN were either spontaneously active (n=13) or quiescent (n=9). Many (62%) of the spontaneously active SPN had distinct patterns of respiratory-related firing (inspiratory (n=3), expiratory (n=4) or pre-inspiratory (n=1)) whereas only 22% of the quiescent cells showed respiratory modulation of excitability. The respiratory modulation was generated by phasic bursts of either excitatory or inhibitory postsynaptic potentials. Most of the spontaneously active SPN were excited by chemoreflex activation (83%), with a shift to post-inspiratory bursting, whereas quiescent SPN were mostly not excited (89%). Diving reflex activation provoked excitation in 75% and 57% of spontaneously active and quiescent SPN, respectively, with only one cell inhibited. Some quiescent SPN (n=4) showed no respiratory modulation under any condition. Spikelet firing (possibly from coupled cells) was seen in four SPN in response to either chemo-stimulation or direct current injection. We have obtained WCR from SPN in a preparation with strong respiratory drive. In this preliminary sample, we have observed a broad range of respiratory modulation in SPN. Using WCR in this in situ preparation, we can examine the relative contributions of intrinsic SPN properties and synaptic inputs that determine the distinct patterns of sympathetic motor activity.