One-component polyurethane sealant specimens were cast between sections of aluminum substrate, cured at standard conditions for 21 days and then subjected to deformations ranging from 50% compression to 75% extension. Specimens were divided into three groups: one group remained at standard conditions, another group was mounted in a xenon arc weathering device and the remaining group was placed in a forced air oven. After 500 hours of exposure specimens were returned to standard conditions and then tested to failure. The sealant characteristics monitored were recovery from deformation, ultimate elongation, tensile strength, strain energy to break and the nature of the failure. Both elongation and strain energy were discriminating indicators of changes in the mechanical properties of the sealant The results indicate that prolonged compression markedly reduced the elongation capacity of the sealant, whereas prolonged extension produced specimens with higher elongation capacities. In most specimens, adhesive failure predominated; cohesive failure was evident in those specimens that had been extended and exposed to elevated temperatures. Five hundred hours of exposure were observed to be insufficient to cause significant chemical degradation of the sealant.