This paper gives a general Monte Carlo procedure for the calculation of perturbations in the reactivity of a multiplying system due to changes in system parameters (e.g., temperature, control rod positions). The procedure is based on the perturbation theory formula which gives the reactivity change δk exact up to terms of second order if the fission density distribution (and) or the corresponding adjoint function are known up to first order. As this formula makes full use of the properties of the importance (adjoint) function, a Monte Carlo procedure is developed for the evaluation of the adjoint function also.