Blood ph, total plasma CO2 and total brain CO2 were determined in the following groups of rats: untreated controls, exposure to 30% CO2, exposure to 50% CO2 and 30 seconds after abrupt withdrawal from 50% CO2. Values were calculated for plasma concentrations of bicarbonate ion and carbonic acid, and for intracellular brain CO2, carbonic acid, bicarbonate and ph. Inhalation of 30% CO2 increased carbonic acid and bicarbonate concentration in plasma and brain cells, and decreased intracellular ph moderately. When the concentration of inhaled CO2 was increased from 30 to 50%, there was a further marked fall in the ph of blood and brain cells, a significant further increase in the amount of carbonic acid in plasma and brain, but no significant further increase in the bicarbonate ion concentration in plasma or brain. On abrupt withdrawal of rats from 50% CO2, the ph of the blood and the brain moved into the range of the ph of the 30% CO2 rats, bicarbonate ion concentration fell below control values; brain bicarbonate ion concentration, however, remained markedly elevated.