1. Stimulation of the rabbit's sciatic nerve produces blood pressure rises. Transections through the brain stem convert most of these pressor reactions into biphasic or depressor responses. 2. the level of transection at which reversal takes place, depends on the frequency of stimulation. the critical plane is located more rostrally for low rates of sciatic stimulation and more caudally for high rates of stimulation. the level also varies from one animal to the other and, in the same animal, it differs for stimulation of the sciatic nerve or of its two major branches. 3. When the full effect of transection has been established, injection of pentobarbitone does not cause any further change. Similarly, after repeated doses of pentobarbitone, when no further qualitative change of vasomotor responses can be produced by additional applications of the anesthetic, brain stem transections are also ineffective. 4. the variability of the effect of brain stem transections is ascribed to the heterogeneous nature of peripheral nerves. the latter carry many different types of sensory fibres, which synapse at various levels throughout the brain stun.