The lethal dose of diphtheria toxin (IDLM per 1 kg of body weight) was injected to Chinchilla line adult rabbits. Histological and electron-microscopy investigations were carried out in time periods from 3 to 24 hours following the administration of the toxin. Degenerative and necrotic changes in the myocardial cells under conditions of the given experiment were not observed. In many muscular cells dilatation of the perinuclear spaces due to the thinning of the myofibrila layer moved to the periphery of the cell was noted. Electron-microscopy investigation of the perinuclear spaces showed a decrease in the number of mitochondria, reduction of the elements of sarcoplasmatic reticulum and disappearance of ribosomes. Cytochemically, in the opened cytoplasmatic matrix there were revealed numerous beta-granules of glycogen. In nuclei fragmentation of nucleoli with reduction of their granular component was observed. The changes described justify the conclusion that acute diphtheria intoxication at early stages of exposure to the toxin brings about impairment of the protein synthesis not only at the level of translation on cytoplasmatic ribosomes, as was suggested before, but at the level of ribosome formation in the nucleus as well.