Articles made from plastic material are likely to change their properties when being used. Oxygen, light and heat when exceeding certain limits bring about chemical alterations which impair the life of the articles. These effects of aging are counteracted by suitable measures, for instance by the addition of stabilizers. Mechanical stress may also affect the life. Due to the cold flow, the permissible stress of thermoplastic materials is the more reduced the longer such articles are exposed to this stress. Extensive research work has given the possibility of establishing maximum permissible stress values in dependence of the time of stress for articles exposed to high mechanical load, as for instance pipes exposed to inner pressure. The present paper deals with the long-term behaviour of pipes made from rigid PVC and from Ziegler-Polyethylene and provides the discussion of methods as to how the reduced permissible stress resulting after prolonged time of exposure may be improved.