Inhalation of 5% carbon dioxide (CO2) and 95% oxygen (O2) or 10% CO2 and 90% O2 at one atm pressure increased blood flow of the C3HBA isotransplant and C3H isoimplant in C3H mice compared with animals breathing 100% O2 or air. Oxygen concentration, measured by a platinum microelectrode, was improved in anoxic areas of the tumor when the animals inhaled the gases containing CO2 compared with those breathing 100% O2 or air; the relative increase was less when the electrode was placed in a well-vascularized area. Rectal temperature, heart and respiration rates were decreased in animals breathing the gases containing CO2 compared with air; rectal temperature was increased by inhalation of 100% O2. The results suggest that inhalation of a gas mixture containing CO2 at atmospheric pressure during radiotherapy or chemotherapy may be of value in treating cancer in humans.