We injured false tendons of the dog heart and examined the intercalated disks (ID) to locate the site of cardiac cell uncoupling. We put the preparations at two calcium concentrations: Ca++-free solution (no healing) and [Ca++]=5.4 mM (‘healing-over’). They were studied immediately, and 1 h, after the injury. We examined the diffusion of Procion yellow from the cut end and measured the relative area of nexus. After the ‘healing-over’ we found that the ID linking damaged and undamaged cells were the barrier to the diffusion of the dye, and that these ID underwent an exponential decrease in the nexal area (time constant 1.5 h). On the other hand, the next ID (i.e. the other extremity of the same cells) kept the nexal area they had in the unhealed condition. These facts indicate that the true uncoupling site is the ID that separate damaged from undamage