This paper presents a method for calculating induced voltage, in vitro, at the terminals of a unipolar pacemaker (PM) subjected to a low frequency magnetic field. We propose a theoretical model which has been experimentally verified by using a homogeneous phantom model placed at the centre of the source generating a homogeneous magnetic field. The levels of the magnetic field used in our experiment are in accordance with the European Directive 2004/40/EC, which sets the occupational electromagnetic field exposure limits. The voltage induced at the terminals of an implanted pacemaker results in the superimposition of two different voltage sources. The first is due to the presence of the loop formed by the PM system and the second is due to the induced currents circulating in the coupling medium. The influence of the induced currents, calculated by the impedance method, is weak compared to the voltage of the loop. The theoretical results obtained agree with the experimental value. Thus, the proposed model can be used to predict the behaviour of a pacemaker subjected to a low frequency magnetic field as well as to those fields within the accepted exposure limits for a patient with a pacemaker.