The requirement for improved understanding of the behaviour of turbine disc alloys at elevated temperatures has led to an increased interest in the contribution of time dependant mechanisms to high temperature fatigue crack growth. A study has been conducted on a new powder alloy to investigate the contribution of such mechanisms when the applied waveform is varied in terms of hold periods and the influence of limited thermal exposure is included. Variable waveform tests performed in air at 725 degrees C have indicated that the addition of a hold time at maximum load in a fatigue cycle tends to increase the crack growth rate per cycle in the as heat treated material. Crack growth in thermally exposed material is retarded by up to a 10 s hold time and then accelerated as the hold time increases further. Rapid near crack tip stress relaxation induced by gamma' coarsening is proposed to have a beneficial effect on the severity of this type of damage which causes the crack growth rate reduction for short hold times.