The deformation behaviour of a C75 in a quenched and tempered state is studied with a focus on thermal stability and strain-rate sensitivity. Compression tests in a wide range of temperatures (room temperature up to 1073.5 K) and strain rates (〖10〗^(-3) s^(-1)<ε ̇<〖10〗^3 s^(-1)) are carried out using a ZwickRoell UP1475 universal testing machine, a 3000 J universal drop tower system, and a Split Hopkinson pressure bar. In addition, microstructural characterization is performed with the help of dilatometric analysis, differential scanning calorimetry and scanning electron microscopy. The strain-rate sensitivity m is calculated from (true) stress-strain curves at fixed levels of deformation (ε = 10 %). Most established constitutive models like those of Johnson & Cook or Zerilli & Armstrong predict only minor increases in strain-rate sensitivity with elevated temperatures. In contrast, the material exhibits a major increase up to m = 0.31 in the temperature range of 773.5 K to 1073.5 K (compared to m = 0.004 at room temperature). The Zener-Hollomon parameter allows to describe the combined effects of both temperature and strain rate on the stress-strain behaviour. Regression analysis is therefore performed using the stress-strain data at high temperatures. The hyperbolic sine equation determines the activation energy for deformation as Q = 281.6 kJ, which is in excellent agreement with literature. While the constitutive approach generally fits the experimental data very well, the surprising increase of the strain-rate sensitivity at low strain rates and at T = 873.5 K cannot be predicted. This surprising finding is discussed in the light of thermal stability and microstructural evolution of the quenched and tempered steel at elevated temperatures.