We deal with the e±cient solution of the so-called bidomain system which is possibly the most complete model for the cardiac bioelectric activity. We study the performance of a non-symmetric structured algebraic multigrid (AMG) preconditioner on the formulation generally used of the bidomain model, i.e. the one characterized by a parabolic equation coupled with an elliptic one. Our numerical results show that, for this formulation, the non- symmetric preconditioner provides the best overall performance compared with the AMG based block structured preconditioners developed in [12]. In this paper we provide theoretical justi¯cation for the observed optimality.