Purpose: CSF fluid dynamics in normal subjects and Chiari I patients have been characterized during rest with Phase contrast MR and Computational Fluid Dynamics (CFD). CSF flow velocities and pressures in the non-resting state have not been adequately characterized. We used computer simulations to study CSF dynamics during increased heart rate in the normal and the Chiari I subarachnoid space. Materials and Methods: Cyclic CSF flow was simulated for multiple cycles in idealized 3D models of the subarachnoid space for normal and Chiari I malformation subarachnoid spaces, with flow cycles corresponding to 80 or 120 heart beats per minute. Flow velocities and pressures were computed by the Navier-Stokes equations. Synchronous bidirectional flow and flow patterns were displayed in Star-CD and inspected visually. Peak velocities and pressure differences in the two models were compared for the two cycle frequencies. Results: Elevating the cycle rate from 80 to 120 cpm increased peak superior-inferior pressure gradients (top-bottom) by just 0.01% in the normal model and 2% in the Chiari model. Corresponding average pressure gradients increased by 92% and 100%, respectively. Also, in both models, the range of synchronous bidirectional flow velocities increased. Systolic velocities had smaller increases with faster cycling. For each cycle rate, peak and average pressure gradients in the Chiari model were greater than in the normal by 11% - 16%. Conclusions: Rising the cycle rate from 80 to 120 cpm increased superior-inferior average pressure gradients and the range of synchronous bidirectional flow velocities in the normal and Chiari I models.