Material properties are one critical requirement for accurate computational modeling of injury events. Modeling brain injuries is complicated by the brain’s heterogeneous structure and measuring material properties is further complicated by the sizes of those structures. We have previously used the atomic force microscope (AFM) to measure pseudo-static mechanical properties of rat brain tissue at a spatial resolution capable of discerning between anatomical structures. We are extending these earlier studies with the AFM to measure frequency dependent properties in response to sinusoidal indentations up to 800 Hz, in some cases. Preliminary analysis indicates significant regional differences in stiffness. Brain tissue stiffness was strongly dependent on both oscillation frequency and indentation depth, reinforcing the need for nonlinear viscoelastic constitutive models to adequately describe the mechanical behavior of brain tissue. The AFM methodology, data analysis, and limitations will be discussed.