Electroencephalograms EEG, electrocardiograms EKG, and respiratory rates were recorded for rats and mice exposed to mixtures of 35 carbondioxide and 65 oxygen. The venous blood, cerebral cortex, cardiac muscle, liver, and skeletal muscle tissues were analyzed for sodium, potassium, chloride, water, carbon dioxide, phosphorus, calcium, magnesium, glucose, hydrogen ion concentration, and adenosinetriphosphate. The EEG initially became slow, followed by hyperexcitability with isolated discharges of combined fast and slow activity resembling the complex of petit mal epilepsy. If convulsions occurred, the electrical activity was symmetrical and bilaterally synchronous. Following seizure, the plasma and cerebral cortex electrolytes shifted in the direction of theoretical hyperexcitable depolarization. Repeated exposures to the carbon dioxide mixtures caused less frequent seizures, often accompanied by cardiac excitement resembling ventricular fibrillation. Unanesthetized, unrestrained rats were used to study susceptability to oxygen poisoning as manifested by convulsions at different pressures. When the exposures were continued beyond the earliest convulsions, the animals developed ventilatory distress and died. The EEG patterns were of the classical seizure type. Exposure to increased pressure and oxygen content caused little change in the EKG. Autopsies revealed no alterations in the brain after oxygen exposure. Spermatogenesis was arrested, but no changes were found in other organs.