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In this study, we used a multi-site optical mapping system to record field-induced responses of single cells isolated from guinea pig hearts. The cells were stained with voltage sensitive dye di-8-ANEPPS and stimulated with two uniform field (S1-S2) pulses along their longitudinal axes. The first pulse (S1=5 ms, <10 V/cm) was applied during rest and elicited an action potential. The second pulse (S2=10 ms, 4-50 V/cm) was applied 15 ms after the break of the S1 pulse (during the action potential plateau). The transmembrane potential responses, V m Fs, were optically recorded from up to 12 sites along the cell length using a fiber optic based optical mapping system at a resolution of 17 or 25 μm. The field-induced V m Fs had a complex spatio-temporal pattern. We show that these responses can be decomposed into simpler components. The first component, termed the differential-mode component (V md F), is like the response of a passive cell. The second component, termed the common-mode component (V mc F), is identical all along the cell and adds a constant offset to the differential mode response of various sites along the cell length, to produce the total V m F responses of the cell.