An implantable medical device with an inductive switching regulator having an inductor with a ferromagnetic core is described. The device incorporates a core saturation detector for detecting saturation in the inductor core indicating the presence of a magnetic field such as produced by an MRI scan. The device is configured to alter its behavior when core saturation is detected such as by entering an MRI mode that may include cessation of therapy, fixed-rate bradycardia pacing, and/or disablement of tachyarrhythmia therapy.