Behavior of specimens produced from one-way polyethylene terephthalate (PET) bottles by milling, drying and injection molding was studied under tensile load. Uniform and stable necking and neck propagation during tensile testing has been replaced under certain tensile rates by an oscillation of stress and neck propagation. Studying the crystallinity and density of specimens undergoing oscillation it was observed that they exhibit more than 50% cavitation. Results of acoustic emission measurements performed during tensile testing revealed that cavitation occurred simultaneously with neck propagation, periodically. Cavities were studied by scanning electron microscopy. In order to study the heat effects coupled with uniform neck propagation heat radiation of the surface was also detected. Infrared thermograms taken by an camera have shown that, within the oscillation cycle, simultaneously with the increase of the stress the temperature of the deformation zone increased, but when the stress decreased (during neck propagation) the temperature decreased.