South America presents several unique geomagnetic features, one of which is the Equatorial Electrojet, a current system which extends east - west in Northern Brazil, for almost 3500 km. Considering the fact that the influence of the Equatorial Electrojet can be detected at great distances from its central axis, it is important to understand its effects on magnetotelluric exploration in Brazil. In magnetotelluric prospecting, the influence of the equatorial electrojet has been modelled for both one and two - dimensional geological media, employing both analytical and numerical techniques solutions such as finite element and finite difference. Three-dimensional geological media have been modeled by "thin – layers "using a "thin sheet" algorithm. Lines of current, gaussian electrojets and undulatory electrojets have been used as induction sources to simulate the equatorial electrojet in these algorithms. In this thesis the principal objective is to model the effects of the equatorial electrojet on three - dimensional structures commonly found in geophysical prospecting. To accomplish this, we have computed numerical solutions of the integral equation for three - dimensional media using the inductive sources mentioned before. As previous works have indicated, our results also show that the influence of the equatorial electrojet is prominant only for frequencies lower than 10-1 Hz. This influence decreases with distance but is detectable up to as far as 3000 km from the center of the electrojet. Thus, the presence of peaks in the apparent resistivity profiles over a homogeneous half - space indicates that the influence of the electrojet is more noticeable for this kind of medium. These peaks display different amplitudes for each type of simulated electrojet, and the peak locations also change from one electrojet to the other. However, when we use more geologically realistic one - dimensional models such as a stratified media, the effect of the electrojet source diminishes considerably and the results do not vary ...