Intracellular zinc concentrations were studied with the aid of an electron microscopic X-ray microanalyzer (EMMA-4) in normal and damaged myocardium and muscle. It was found that nuclei of all cells, but especially those of polymorphonuclear leukocytes, contain high concentrations of zinc. Where nucleoli were prominent, they usually contained more zinc than other parts of the nucleus. Granules of mast cells, eosinophils and macrophages were rich in zinc while neutrophils or heterophil granules, residual bodies, fat droplets, nerve axons and interstitial ground substance were not. The changes following injury involved an overall increase in zinc concentration in damaged areas. Degenerating cell nuclei (notably the polymorphonuclear and myofiber nuclei) lost their zinc, while interstitial cells of various kinds accumulated it. High zinc concentrations after injury occurred in mitochondria of degenerating myofibers and phagosomes of histiocytic macrophages. The possibility of employing X-ray microanalysis in more detailed studies of zinc activity in reaction to injury was suggested.