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Department of Physiology, Royal Free Hospital School of Medicine, London, UK. JFXJONES@iveagh.ucd.ie

1. The reflex changes in the dromotropic state of the heart (P-R interval or atrioventricular conduction time) in response to selective stimulation of the carotid and aortic bodies by sodium cyanide were studied in the anaesthetized cat. The heart was paced and the arterial blood pressure was kept constant to minimize secondary effects of changes in arterial baroreceptor activity. 2. Stimulation of the carotid and aortic bodies caused an increase in the atrioventricular conduction time. 3. Evidence is presented to suggest that this negative dromotropic response was due predominantly to a vagal cholinergic mechanism. There is a small sympathetic component but only in so far as the carotid body reflex is concerned. 4. The negative dromotropic responses were enhanced during reflex suppression of the central inspiratory neuronal drive combined with minimal activity of the slowly adapting pulmonary stretch afferents indicating that they are respiratory modulated. 5. The clinical implications of these results are discussed.