Melt-spun polypropylene fibres with various molecular weight distributions (MWDs) and processing parameters were subjected to stress relaxation tests at 23 and 100 degrees C. The average level of the relaxation curves was mainly determined by processing conditions, but the MWD affected the relaxation kinetics substantially, especially at 100 degrees C. In general, the relaxation rate was reduced by increasing the molecular orientation, increasing the degree of crystallinity, and broadening the MWD. However, the effect of the MWD was reversed for the initial relaxation at 100 degrees C. MWD effects at 23 and 100 degrees C were in accordance with relaxation spectra obtained by dynamic mechanical analysis. Stress plateaus at long relaxation times were observed for broad MWDs at 100 degrees C. The presence of a crystalline structure seems to be a prerequisite for both the 'reversed' MWD effect and the stress plateaus. The observations reported in this study can be interpreted in terms of molecular reptation, molecular constraints and structural relaxation.