The contribution focuses on the response simulation of friction pendulum seismic isolators during a strong ground motion. The device is considered an isolated structural element. Its action is effectively described as the motion of a particle (i.e., the slider) on a spherical surface (i.e., the sliding surface). The bearing is inherently a system with three degrees of freedom and a Lagrangian formalism is employed to derive its equations of motion. Taking into account that the slider is constrained to move only on the spherical sliding surface, one of the degrees of freedom remains constant when the bearing is activated. Two second-order ordinary differential equations can be consequently derived for the remaining degrees of freedom. The response of the isolator depends strongly on the friction coefficient. The latter doesn’t remain constant, it is assumed to vary with the sliding velocity, the normal pressure acting on the interface, and the rise in the temperature during sliding. The input for the analyses is provided in terms of acceleration time series recorded during a real earthquake. Numerical simulations are performed and results obtained in terms of slider’s trajectories are presented.