In this paper, we study the stability result for the conductivities diffusion coefficients to a strongly reaction-diffusion system modeling electrical activity in the heart. To study the problem, we establish a Carleman estimate for our system. The proof is based on the combination of a Carleman estimate and certain weight energy estimates for parabolic systems. 1. Introduction. Let Ω ⊂ R N (N ≥ 1) be a bounded connected open set whose boundary ∂Ω is regular enough. Let T > 0 and ω be a small nonempty subset of Ω. We will denote (0, T) × Ω by Q T and (0, T) × ∂Ω by Σ T. To state the model of the cardiac electric activity in Ω (Ω ⊂ R 3 being the natural domain of the heart), we set u i = u i (t, x) and u e = u e (t, x) to represent the spacial cellular and location x ∈ Ω of the intracellular and extracellular electric potentials respectively. Their difference v = u i − u e is the transmembrane potential. The anisotropic properties of the two media are modeled by intracellular and extracellular conductivity tensors M i (x) and M e (x). The surface capacitance of the membrane is represented by the constant c m > 0. The transmembrane ionic current is represented by a nonlinear function h(v). The equations governing the cardiac electric activity are given by the coupled reaction-diffusion system: c m ∂ t v − div(M i (x)∇u i) + h(v) = f χ ω , in Q T , c m ∂ t v + div(M e (x)∇u e) + h(v) = gχ ω , in Q T , (1) where f and g are stimulation currents applied to Ω. We complete this model with Dirichlet boundary conditions for the intra-and extracellular electric potentials