[en] A commercially available HP-Si₃N₄ has been exposed for 200 hours in a 0.4%H₂S+0.75%H₂O+H₂ bal. environment at 1200 and 1300 C. Subsequently long term constant stress tests have been performed at the exposure temperatures in the uniaxial tensile mode. Strength, toughness and subcritical crack growth tests results obtained in the flexural loading mode presented elsewhere highlight the effect of a reduction in the amount in the intergranular glassy phase. However, the interpretation of flexural test results at high temperature is hampered by time-dependent changes in the stress distribution and a shift of the neutral axis, making it very difficult to derive the constitutive equations without making arbitrary assumptions. This paper therefore deals with the comparison of the long term uniaxial tensile mechanical behaviour of the material in both as-received and exposed conditions. In order not to mask the intrinsic behaviour of the exposed material, all tests are performed in vacuum. (orig.)