The aim of this paper, describing a semi-active Mass Damper system based on Magnetorheological (MR) Devices, is the evaluation of the effectiveness of a vibration control system based on semi-active dampers, applied to the case of a typical Italian historical construction subjected to seismic action. The reference model has been extracted from the structural scheme of a long-bay building and the control strategy takes into account a mass damper system. The MR damper design has been dealt with, by considering mechanical, hydraulic and electronic related aspects and problems. A specific logical scheme of the building integrated with the MR device has been analysed by means of the Simulink toolbox. By taking advantage of this model, the performance of the semi-active control system has been evaluated in comparison with the passive control strategy and with the structure without control.