We present a 3-D model for the simulation of a realistic clinical situation during magnetic stimulation. The electromagnetic problem is solved by reconstructing the inhomogeneous head tissues from magnetic resonance images and associating relative values of conductivity to each tissue. Application of Maxwell's equations in the integral form leads to an equivalent 3-D electrical network, whose solution gives the current density distribution in the brain. The high spatial resolution and rigorous electromagnetic approach make this model an accurate and useful tool for stimulator design and for estimating the efficient and safety of this clinical methodology.