The transition from the glassy state to the rubbery state in a partially oriented high-impact polystyrene (HIPS) sample has been investigated by means of dynamic mechanical analysis (DMA), using three-point bending and parallel plate compression test modes. For samples without any thermal treatment an apparent increase of the storage modulus, E'(T), was observed near the onset of the calorimetric glass transition temperature, followed by a strong decrease due to the dynamic glass transition. This peak was attributed to the stress release of the oriented material and almost disappeared for an annealed sample. The compression experiments were found to be more sensitive to the stress release phenomena than flexural experiments. Several experiments at different frequencies (from 0.5 to 20 Hz) showed that both the area and the height of the E'(T) peaks increase linearly with the logarithm of frequency. The influence of the heating rate (for a fixed frequency - 1 Hz) was also analysed and the thermal calibration method used prior to those experiments is discussed. It was found that the strength of the E'(T) peaks decreases with increasing heating rate and the corresponding loss peaks, E "(T), shift to higher temperatures and become sharper.