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1. A fluorescent tracer dye, sodium fluorescein (mol.wt. 332), was used to assess the relative degree of intercellular coupling in various tissues of the rabbit heart. 2. Dye was injected intracellularly by micro-iontophoresis. Subsequent movement into contiguous cells was monitored by video microscopy. From these data the permeability of the intercellular boundaries was computed. 3. The values of boundary permeability were consistent with those expected from previous studies with tracers whose molecular weights bracketed that of fluorescein. 4. In the atrium, ventricle, Purkinje strands and His bundle, the relative magnitude of the boundary permeability correlated reasonably well with the relative profusity of gap junctional area on the intercalated disk, the latter estimated from published data. 5. The rate of passage of dye between N cells of the atrioventricular, AV, node was at least three orders of magnitude lower than between cells of the other tissues studied; this result is consistent with published reports indicating few gap junctions between cells within the region of slow conduction. 6. Quantitative considerations based on these data indicate that N cells may not be sufficiently well coupled to permit impulse propagation through the AV node by intercellular current flow, alone.