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For evaluation of thermal endurance in foamed plastics, the temperature and time dependence of the compression dynamic modulus of seven flexible polyurethane foams were investigated by dynamic viscoelastic measurements. In a certain temperature range, the compression dynamic modulus of each polyurethane foam lessened gradually with time, presumably due to a relaxation phenomenon of foam skeletons accompanying deterioration in the bending stiffness of the skeleton. The logarithm of time t, by which the compression dynamic modulus fell to 50% of its initial value, has been found to be a linear function of reciprocal temperature T-1. The parameter A is the slope of this function and has been found to correlate negatively, with the porosity but with the hardness positively. [ABSTRACT FROM AUTHOR]