This study was motivated by the RF, microwave and antenna industries to include passive and active circuits with measured S-parameters in the FDTD simulation. Recently, a passivity enforcement method based on an inverse eigenvalue method was proposed for transient circuit simulation (C.Saunders and M.Steer, IEEE Trans. Microwave Theory Tech., vol.99, no.11, 2011). In this paper, the passivity enforcement method is introduced to the electromagnetic simulation method FDTD to simulate circuit networks characterized by measured S-parameters together with other geometries such as antennas. Using XFdtd®, the simulation results for some examples showed that various methods, such as the Laplace transform, combined with the passivity enforcement method were quite accurate and robust. This combined FDTD and circuit method can be applied to simulate antennas together with these devices characterized by measured S-parameters.