This paper presents an electric circuit model and a dynamic analysis of enzymatic biofuel cells. The model is consistent with classical double-layer capacitance electrode behavior, fuel cell polarization models, and fuel diffusion limits, and may be extracted from commonly used electrochemical measurements. It is shown to accurately predict the observed experimental behavior of implantable enzymatic biofuel cells operating in vivo. The model is analyzed under various power loading conditions to consider runtime and fuel replenishment implications. A case study for powering a pacemaker is considered; and the results and SPICE simulations are shown to be in excellent agreement with experimental observations. The model can be used to identify areas for future biofuel cell improvement and to provide insight into critical electrical interface and system-level issues that must be addressed to advance the adoption of in vivo application of biofuel cells.