Representation of cardiac tissue by a continuous electrical cable provides a simple tool to explain impulse propagation and to make a comparison between heart, skeletal muscle and nerve. Recent experimental and theoretical studies have shown, however, that the process of electrical impulse propagation in heart is complex, due to the presence of cell borders and septa of connective tissue. At sites where propagation deviates from a linear profile, action potential generation gets delayed, and in cases of decreased excitability, unidirectional block may occur. At such sites, propagation is carried by the slow Ca++ inward current, in addition to the rapid Na+ inward current. As a consequence, local propagation may become sensitive to inhibition of Ca++ channels. Moreover, computer simulations have shown that electrical cell-to-cell uncoupling an gap junctions can reverse unidirectional block at such sites to bidirectional conduction. This complex interaction between function and structure which is likely to play a major role in remodeled tissue (hypertrophy, chronic infarction) has to be taken into account in the evaluation of the mechanisms of action of antiarrhythmic drugs.