By means of finite-difference time-domain (FDTD) simulations, the stationary and dynamic responses of coupled optical microring resonators made with a material exhibiting an instantaneous Kerr nonlinearity are numerically investigated. We compare the results with the coupled-mode theory (CMT) and find good agreement. We demonstrate by integrating Maxwell’s equations that this system can show a self-pulsing operation in the normal dispersion regime. Finally, this work shows that FDTD simulations can be combined with a CMT analysis for the purpose of designing optical functions whose operation is based on nonlinear coupled micro-resonators.