✓ Seizures were induced in 12 monkeys rendered hemiparetic by middle cerebral artery occlusion. In a control group of five hemiparetic monkeys seizures were not induced. A mass spectrometer was used to monitor regional oxygen tension (pO2), carbon dioxide tension (pCO2) and the partial pressure of argon (pA) within the ischemic brains. Seisure activity resulted in a 74.3% mean increase in pO2 and 16.9% mean decrease in pCO2. The mean argon washout time was decreased 29.1%. The changes were transient and recovery from hemiparesis was no better in the experimental group than in the control group.