The design of electrical impedance spectroscopy systems is driven by target applications. Moreover, system functions are interdependent. Thus, optimizing the design of each block without input from the others is impossible. Furthermore, simulating the whole system at transistor level is time consuming. This chapter presents a mixed-level simulation tool for the efficient development of bioimpedance spec-troscopy measurement systems. We have developed Verilog-AMS models combined with transistor-level descriptions of several analog blocks. This allows us to run highly efficient system-level simulations for a whole circuit. We apply this tool to the design of an integrated circuit suitable for in vivo monitoring of bluefin tuna physiological parameters. In fact, the variation of biological processes offers significant insights into a host of key parameters such as growth, survival and reproduction. The obtained results show that the approach described in this chapter can reduce simulation time by 80 % while consistently reaching high simulation performances.