ABSTRACT
Acute "pontine" cats were submitted to abrupt increases in blood pressure induced by a transient obstruction of the lower thoracic aorta. This stimulation regularly provoked a complex electrocortical response. The first part of it, which occured 12 sec after the beginning of the pressure rise, consisted of an activation reaction accompanied by dilatation of the pupils and tonic contraction of the lower back muscle and preceded by arrest of respiration. The activation reaction and pupillary dilatation were absent in the cats with a brain stem section at the junction between the pons and the mesencephalon. The effects on the cerebrum are attributed to a passive increase in blood flow which is effective on ECoG and pupils only when the midbrain is connected to the brain. The second and third parts of the response, a burst of slow waves followed by a flattening of the record, were dependent of the hypotension which followed the end of the blood pressure rise. They were preserved in the cats with a lesion of the junction between the pons and the mesencephalon. They are attributed to a relative ischemia of the brain.