1. The onset and time course of baroreceptor inhibition of pre- and post-ganglionic sympathetic reflex activity has been examined in the anaesthetized cat.2. The shortest time to the onset of inhibition of an intercostal evoked reflex response in cardiac and renal nerve was less than 90 msec following a rise in pressure in a carotid sinus blind sac, and around 55 msec following stimulation of the ipsilateral sinus nerve. The cardiac nerve response was completely inhibited before the renal nerve response.3. Because of the long delays in the somato-sympathetic reflex pathway it is argued that these minimum times will be much less than the real central delay of baroreceptor inhibition. These were estimated by adding on the central times for the somato-sympathetic reflexes to give latencies of 94-143 msec for the inhibition.4. A spinal sympathetic reflex was inhibited by 30-75% following a rise in pressure in a carotid sinus blind sac or sinus nerve stimulation. The minimum time for this inhibition was around 100 msec.5. The baroreceptor inhibition of the spinal sympathetic reflex was abolished following section of a restricted region in the dorsolateral part of the lateral funiculus of the cervical spinal cord.6. Both pre- and post-ganglionic reflexes could be inhibited when stimulating within three regions of the medulla oblongata. The latency to inhibition elicited from the ventromedial reticular formation was short, some 5-30 msec, whereas that elicited from a ventrolateral region or the mid line raphe nucleus was long, some 90-160 msec.7. The possibility is discussed that the baroreceptor inhibition of both the pre- and post-ganglionic reflexes examined in this study is occurring at the spinal level via a pathway from either the raphe nuclei or ventrolateral medulla.