By combination of light, electron-microscopical and histochemical findings in the myocardium of dog and mice, the hypothesis of an intimate contact between a widely branched interstitial cell type, rich in proteoglycans, and myocardial cells could be strengthened. The branches seem to penetrate the transversal tubular system of the myocardial cells. Functionally, this interstitial (mast cell-like) cell type might influence via proteoglycans – delivered into the transversal tubuli – the excitation phenomena of the myocardial cell membrane. One of the earliest pathomorphological changes in the myocardium, as revealed by the light-microscope, is the disengagement of these interstitial cells and the myocardial cells. From this point of view the genesis of myocardial cell necrosis has been discus