Heart rate variability is largely dependent on cardiac vagal activity and its loss is an independent risk factor for arrhythmias and cardiac mortality. Thus, it is important to elucidate the factors that modulate cardiac vagal ganglionic transmission in health and in cardiovascular disease states. Although cardiac vagal ganglionic transmission has been studied in a number of in vitro preparations (Edwards et al 1995) there have been no intracellular recordings from cardiac ganglion neurones in a functionally intact setting to permit the detailed analysis of synaptic integration and modulation at this site. Using the working heart-brainstem preparation (Paton, 1996) from Wistar rats (aged 4-5 weeks, initially anaesthetised with Halothane), we have obtained the first intracellular recordings from functionally connected cardiac vagal ganglion cells in situ. The atria were dissected from the ventricles, pinned flat and stabilized with a nylon mesh foot. Intracellular recordings of 33 cardiac ganglion cells were made with sharp microelectrodes (0.5M KCl; 80-120 MOhm). Stable recordings were obtained for periods of over 30 minutes allowing examination of intrinsic properties, spontaneous firing pattern and responses to vagus nerve (right) evoked and cardiorespiratory reflex activation: peripheral chemoreflex (NaCN, 0.03% i.a.; baroreflex, by increasing pump flow; diving response, 10°C saline applied to snout. Several different classes of neurone were identifiable on the basis of their intrinsic electrophysiology, reminiscent of Edwards et al. (1995), and distinct patterns of spontaneous and reflex evoked activities. Active neurones showed EPSPs and/or action potentials (AP) that occurred most commonly in the post-inspiratory phase. In these neurones activation of bradycardic reflexes (baro-, chemo-, nasotrigeminal) increased EPSP frequency and/or AP with atrial rhythm slowing. Spontaneous, vagus- and reflexly- evoked AP appeared mostly triggered by suprathreshold unitary EPSPs rather than by summation of subthreshold EPSPs. Vagus stimulation evoked EPSPs at a latency of 30-40 ms. In conclusion, it is possible to obtain high resolution, stable intracellular recordings from physiologically intact cardiac vagal ganglion cells, which exhibit appropriate patterns of ongoing and reflex excitatory synaptic drives. Future studies will elucidate how cardiac vagal transmission may be modified in health and disease.