This paper presents a hybrid finite-difference time-domain-simulation program with integrated circuit emphasis (FDTD-SPICE) method for lossless transmission lines excited by a nonuniform electromagnetic field. An FDTD code is used to compute the excitation fields of the lines. Additional voltage sources, which denote the effect of the excitation fields, are computed with the results obtained by the FDTD method and incorporated into a SPICE model. The hybrid method can be used for nonlinear or time-varying loads, because the SPICE model allows easy incorporation of nonlinear or time-varying loads. Three cases, two transmission lines and a network, all inside a cavity excited by an incident electromagnetic field, are computed by using the presented method. The numerical results are compared with those obtained by using a 3D FDTD code, and the agreement is generally good. The time needed by the hybrid method is much less than that needed by the FDTD code, because the lines can be ignored when the excitation fields of the lines are computed in the hybrid method.