The effects of acute carbon monoxide poisoning on the ultrastructure of the muscle had been investigated. Dogs were exposed to 10.1 vol.% carbon monoxide by intracoronary perfusion for 10, 20 and 30 min. The ultrastructural changes and cytochrome-oxidase activity reduction were evident after 10 min and were most prominent after 20 and 30 min. Degenerative changes of mitochondria, pericapillary and interfibrillar edema, sarcoplasmic reticulum dilatation, glycogen content decrease and in local areas a total or partial necrosis of myofibrils could characteristically be observed. The cytochrome-oxidase and glycose-6-phosphate dehydrogenase showed a significant decrease. A dialysis concentrate from deproteinised calf blood (Solcoseryl, Actihaemyl) proved to inhibit morphological and enzymatical pathological changes; the destruction was slowed and prevented. On this basis the protective effect of Solcoseryl may be of considerable importance.