The flexural strength of yttria-partially stabilized zirconia was evaluated as a function of temperature (20-1000 degrees C in air), applied stress and time. The material was susceptible to strength degradation at low temperatures (200 and 300 degrees C) due to the phase transformation of the tetragonal structure to monoclinic, possibly accompanied by microcracking. In this temperature range, the material was incapable of sustaining low applied stress levels of 276 MPa for any significant duration (> 100 h < 500 h). Stress rupture testing at 600 degrees C and above identified the onset of viscous flow of the glassy phase and consequent : degradation of material strength.