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1. The interaction between the pressor response to electrical stimulation of the fastigial nucleus (FN), the fastigial pressor response (FPR), and the depressor response to electrical stimulation of the carotid sinus nerve (CSN) was examined in paralysed anaesthetized cats.

2. Blood pressure responses evoked by electrical stimulation of the FN and the CSN were mutually inhibitory and summed algebraically.

3. The FPR was augmented after denervation of buffer nerves. Lesions of the FN did not alter the depressor response to stimulation of the CSN.

4. Bilateral electrolytic lesions of the paramedian reticular nucleus abolished both the FPR and the CSN depressor response without altering base line pressure.

5. With micro-electrode recording neurones were discovered within the paramedian reticular nucleus which responded to electrical stimulation of the FN or the CSN. These neurones were polysynaptically excited by stimulation of either the FN or the CSN but rarely from both, and could be further subdivided into cells responding with either a single spike or a burst discharge.

6. The interaction between the FN and the CSN projections to the paramedian reticular nucleus was examined by conditioning-test studies. Eleven per cent of FN- and CSN-units were inhibited by conditioning stimulation of the heteronymous input. The interaction was exclusively inhibitory and observed only in units with latencies > 4 msec and having burst responses. The latency for inhibition was > 20 msec, peaked around 100 msec and lasted up to 300 msec.

7. We conclude that the FRP is buffered by baroreceptors and that there is a mutually inhibitory interaction between projections from the FN and the CSN acting on sympathetic vasomotor neurones. The paramedian reticular nucleus appears to be an important site for the interaction.

8. The findings support the view that interneurones mediating pressor and depressor responses are intermixed within the medial reticular formation of the medulla.