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The effects of graded hemorrhagic hypotension on electrocortical function was investigated in 12 cats anesthetized with a-chloralose. Cerebral function was assessed both in terms of spontaneous activity (EEG) and the somatosensory evoked response. No significant changes in the EEG trace or in the amplitude of the positive/negative waves of the primary evoked response were observed at mean arterial pressures of between 120 mm Hg and 40 mm Hg. At levels of systemic arterial pressure of less than 40 mm Hg cortical rhythms became slower as pressure was decreased and an isoelectric EEG was recorded in the pressure range 10-30 mm Hg. The earliest sign of any change in the amplitude of the primary evoked response was observed at arterial pressures of approximately 35-40 mm Hg. Below this value the amplitude decreased with decreasing systemic pressure and became zero in the pressure range 15-25 mm Hg.