In this paper we introduce a new distributed algorithm for the exploration of an unknown environment with a team of mobile robots. The objective is to explore the whole environment as fastest as possible. The proposed approach is based on the potential field method. The advantages of using this method are several and well known, but the presence of many local minima does not assure the exploration of the entire environment. Our idea is to preserve these advantages but overcome the problem of local minima by introducing a leader in the team which has a different control law, unaffected by this problem. Furthermore, we consider also the case of several local leaders, dynamically selected on the basis of a hierarchy within the team. Extensive simulations are presented to evaluate the performance of the algorithm. In particular, the results are compared with the exploration obtained by a potential field approach without leaders.