A system including a sensor and electronic instrumentation for measuring fission gas pressures inside a pressurized system or device or the like, such as a nuclear fuel rod. The system is based primarily on what is commonly known as the Villari effect which is the dependence of the magnetic permeability of certain magnetic materials upon applied stress, and permits pressure measurements to be made without any mechanical or physical penetration of the fuel cladding or end cap of the fuel rod. In a preferred embodiment pressure sensing is implemented by locating a sensor made from Villari sensitive material inside the pressurized system. The pressure places a stress loading on the sensor which in turn produces electrical output signals proportional to the applied stress. By sensing the initial magnetic permeability of the sensor, the difference between that sensed information and subsequently sensed similar information can be obtained, thereby providing an indication of any change in system pressure and the degree of such change. Such measurements can be made sequentially at multiple locations by placing an external electromagnetic search coil adjacent the internal sensor in the pressurized system such that the magnetic field of the search coil penetrates the containment wall of the pressurized system, thus interrogating the sensor and providing an appropriate readout which can be calibrated in terms of system pressure.