Composite materials are ideal for many weight-conscious applications such as aerospace and automotive structures because of their exceptionally high specific properties. However, composite materials are susceptible to complex damage and difficult-to-predict damage growth. This necessitates the application of structural health monitoring (SHM) for in-operation monitoring of damage formation and accumulation. Self-sensing materials are strong candidates for composite SHM because they do not suffer from limitations associated with traditional, point-based sensors. A common approach to self-sensing is the piezoresistive effect in nanofiller-modified materials. To date, work in the area of self-sensing via the piezoresistive effect has focused overwhelmingly on the direct current (DC) response of these materials. This is an important limitation because alternating current (AC) effects inherently provide more information by relating both impedance and phase to damage. Therefore, this work explores the effect of high-cycle fatigue loading on the AC response of carbon nanofiber (CNF)-modified glass fiber/epoxy laminates. Specifically, impedance magnitude and phase angle are both measured through the thickness and along the length of a tension-tension fatigue-loaded specimen with an open-hole stress concentration as a function of load cycle and up to 10 MHz. The collected impedance data is then fit to an equivalent circuit model and correlated to stiffness changes. This means that changes in equivalent circuit behavior can be used to track fatigue-induced softening in self-sensing composites. In light of these promising preliminary results, AC effects appear to have considerable potential for real-time tracking of damage accumulation.