Electrical coupling between myocytes plays a critical role in propagation, repolarization, and arrhythmias. On the basis of predictions from cable theory, we hypothesized that the cardiac space constant (λ) measured from the decay of subthreshold transmembrane potential (ST-V _m) in space would provide an index of regional cell-to-cell coupling in the intact heart. With the use of voltage-sensitive dyes, the distribution of ST-V _m was measured from hundreds of sites in close proximity to the site of subthreshold stimulation. λ was calculated from the exponential decay of ST-V _min space. Consistent with known directional differences in axial resistance, the spatial distribution of ST-V _mwas strongly dependent on fiber orientation, because λ was significantly (P < 0.001) longer along (1.5 ± 0.1 mm) compared with across (0.8 ± 0.1 mm) fibers. There was a close linear relationship (P < 0.001) between conduction velocity (CV) and λ along all fiber angles tested. Reducing gap junctional conductance by heptanol reversibly decreased CV and λ in parallel by ∼50%. In contrast, sodium channel blockade by flecainide slowed CV by 40% but had no effect on λ, reaffirming that λ was an index of passive but not active membrane properties. These data establish the feasibility of measuring λ as an index of cell-to-cell coupling in the intact heart, and indicate strong dependency of λ on fiber orientation and pharmacological alterations of gap junction conductance.