The effect of high current (up to 107 A/cm2) and temperature stressing (up to 300°C) in vacuum (10−8 Torr) and air, on the properties of evaporated 200 A° thick permalloy magnetoresistors has been investigated. Tests in air have established that sensors are most likely to fail due to oxidation and the associated degradation of their magnetic properties. In contrast, no detrimental changes were observed for devices tested in vacuum for 800 hours, at 107 A/cm2, and 250°C. Device lifetimes at 85°C and J ≤ 10 7 A/cm 2 are estimated to be only of the order of 1 year in air and more than 103 years in an inert atmosphere.