SUMMARY1. These experiments were designed to confirm that at the onset of treadmill exercise in rabbits the tonic reflex depressor effects of input to the central nervous system from arterial baroreceptors is abolished, thus contributing to the rise of systemic arterial pressure (SAP) and heart rate (HR).2. An inflatable cuff was placed around one common carotid artery after the remaining arterial baroreceptors had been surgically denervated. Transient inflation of the cuff caused reflex rises of SAP and HR, which were much reduced during the first minute of exercise. Deflation of the cuff caused a brisk fall of HR, which was completely abolished by exercise.3. A snare was placed around one carotid sinus nerve after the remaining arterial baroreceptors had been surgically denervated. Where the snare was tightened all arterial baroreceptor reflexes were immediately and permanently abolished. This allowed the reflex effects of baroreceptor input to be calculated by difference. The magnitude of these calculated effects, at rest and during exercise, diminished according to how long after barodenervation the observations were made.4. We conclude from the above experiments that the resting tonic reflex depression of SAP and HR caused by baroreceptor input is much reduced, rather than completely abolished, at the onset of exercise.5. We also conclude, from the effects of partial surgical barodenervation, and of unloading the carotid baroreceptors prior to exercise by inflating the carotid cuff, that resting input from the arterial baroreceptors must be near zero before the cardiovascular response to exercise is grossly altered.