Effects of various concentrations of CO2 on brain excitability and electrolyte distribution in rats were studied, and also some properties of seizures induced by abrupt withdrawal from high concentrations of CO2. Inhalation of relatively low concentrations of CO2 (5–20%) decreases brain excitability, as measured by an increase in electroshock seizure threshold (EST). In moderately high concentrations (25–40%), CO2 increases brain excitability, as measured by a decrease in EST and the appearance of spontaneous seizures. Inhalation of high concentration of CO2 (40% or higher) markedly decreases brain excitability and causes anesthesia. Thus the effect of CO2 on brain excitability is related to the concentration inhaled. Abrupt removal of rats from high (anesthetic) concentrations of CO2 results in spontaneous clonic seizures within 30 seconds to 1 minute after withdrawal; these seizures last for 1–2 minutes. Inhalation of 50% CO2 decreases brain intracellular Na and K concentrations and produces a marked cellular acidosis. Thirty seconds after abrupt withdrawal of rats from 50% CO2, but prior to the onset of seizures, the concentration of Na in brain cells increases and the concentration of K decreases.

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