Organization of cardiac tissue into cell strands and layers has been implicated in changes of transmembrane potential (ΔV(m)) during defibrillation. To determine the shock-induced ΔV(m) in such structures, cell strands of variable width [strand width (SW) = 0.15-2 mm] were grown in culture. Uniform-field shocks with variable strength [shock strength (SS) = 2-50 V/cm] were applied across strands during the action potential (AP) plateau, and ΔV(m) were measured optically. Three different types of ΔV(m) were observed. Small ΔV(m) [<40%AP amplitude (APA)] were linearly dependent on SS and SW and were symmetrically distributed about a strand centerline with maximal positive and negative ΔV(m) on opposite strand sides being equal. Intermediate ΔV(m) (<200%APA) were strongly asymmetric with negative ΔV(m) > positive ΔV(m) because of a negative time-dependent shift of V(m) at the depolarized side of the strands. For large ΔV(m) (>200%APA), a second time-dependent shift of V(m) to more positive levels was observed in the hyperpolarized portions of strands, causing reduction of the ΔV(m) asymmetry. We conclude that during application of shocks to cell strands during the AP plateau, passive changes of V(m) were followed by two voltage- and time-dependent shifts of V(m), possibly reflecting changes of ionic currents or membrane electroporation.