Abstract A preliminary study concerning the responses of a Generation IV (GEN-IV) Lead Fast Reactor (LFR) demonstrator (DEMO) core to externally-induced reactivity perturbations has been carried out with the aim at assessing and comparing the dynamic performances of MOX and metallic fuel alternative options. Reactivity coefficients and kinetics parameters have been calculated for both Beginning of Cycle (BoC) and End of Cycle (EoC) configurations by means of ERANOS deterministic code ver. 2.1 associated with the JEFF-3.1 data library. A simplified lumped-parameter model has been developed to treat both neutronics (point-kinetics approximation) and thermal-hydraulics (average temperature heat-exchange model); the latter have been then coupled to analyze MOX and metallic fuel behaviors following operational transient initiators, such as control rod partial extraction (reactivity insertion), coolant inlet temperature perturbation (simulating a loss of heat sink), and mass flow rate reduction (loss of flow), using the MATLAB/SIMULINK® tool. The analysis of DEMO core sub-system open-loop stability has ultimately been performed. Results have shown that the model is stable and evidences a satisfactory capability of predicting the response to the reactivity perturbations considered.