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Fourteen late juvenile monkeys were subjected to a single 30-min episodes of markedhypotension using an infusion of trimethaphan. Afterward, blood pressure was rapidly restored to and maintained at preinsult levels with an intravenous drip of phenylephrine (.02 mg/ml). The respiratory gas tensions and pH of the arterial blood weremaintained within normal limits at all times. During the 30-min episodes, the cortical electrical impedence increased by a mean of 27% while the EEG frequencies and amplitudes decreased by 47 and 30%, respectively. The impedance and EEG activity generally altered at the same time but inversely to one another, although, on restoration of blood pressure, the impedance recovered within minutes while the EEG activity returned to normal only after 1 h. The impedance values and EEG activity of those animals that eventually died began to diverge significantly from the values of theanimals that were to survive at about 2.5 h after restoration of blood pressure.