The process of deformation of twinned ferromagnetic martensite under a combined action of slowly increasing magnetic field (or mechanical stress) and oscillatory mechanical stress has been studied theoretically in the framework of a statistical model of magnetically/mechanically induced deformation. The dependencies of magnetically/mechanically induced deformation on time, oscillatory stress amplitude and magnetic field (or mechanical stress) has been modeled. The additional deformation of a Ni-Mn-Ga specimen activated by a superimposed oscillatory stress has been computed for the case of resonant longitudinal ultrasonic waves with several realistic values of strain amplitudes using the typical values of Young's modulus, specimen length, and threshold field/stress (i.e. the field/stress, which triggers the process of magnetically/mechanically induced deformation). Quantitative estimations show, in particular, that the ultrasonic wave with a strain amplitude of 4 Â 10 À5 causes a $30% decrease of the threshold field/stress value; the additional deformation induced by the ultrasound reaches its maximal value %2.8% when the field/stress reaches the initial (i.e. observed in the absence of ultrasound) threshold value.