The stress-strain curves and stress-relaxation curves of polypropylene are obtained by using a closed-loop, electrohydraulic, servo-controlled testing machine. Effects of mean strain changes on deformation behaviour are examined in a tension-compression mode under strain control at room temperature (18-23 degrees C). The hysteresis loops of three mean strains show a steady-state response from the stress-strain curves at a strain rate of 1 x 10(-3) s(-1), at a strain width of 5%, at a number of cycles of N = 50 and at three mean strains (epsilon(m) = 0, +1.0 and +2.0%). The drop of stress at the mean strain of epsilon(m) = -1.0% is larger in magnitude than that at epsilon(m) = +1.0%; this is caused by the higher stress level at epsilon(m) = -1.0% as compared with the stress level at epsilon(m) = +1.0%. From the results of stress amplitude and the stress drop behaviour, the magnitude of stress drop is hardly affected by the mean strain.