Rolling-bearing cages (separators) which were made by injection moulding from polyamide 66 filled with short glass fibres were tested before and after exposures in hot air at temperatures 120, 140, 160, and 180 degrees C at various times ranging from a few hours to three weeks. The cages were loaded in tension and the changes of maximum force corresponding to ultimate tensile stress were studied. A very simple degradation mechanism containing the main features of the real degradation processes was proposed and its kinetic equations were derived. On the basis of the assumption that the changes of properties during ageing are proportional to the changes of chemical composition and structure a regression function was proposed. It was used for the fitting of the measured results and for the construction of predictive curves which describe the temporal and temperature dependence of strength for working temperatures and long times.