Discusses the natural ventilation of the Paintings Room of Altamira Cave, in Spain. The radon-222 concentration was measured by scintillation cells of capacity 500 cm3. From the mean monthly Radon-222 concentration the natural ventilation was calculated using Wilkening's method. Compares the variations in ventilation rate (monthly) with the temperature differences between the Paintings Room and the Hall Chamber. Natural ventilation is the major factor affecting changes in the carbon dioxide concentration.