Long-term hemodynamic overload of the heart leads to an increase in myocardial mass. In most cases it is not known to what degree the single components in the myocardium (water, protein, nonprotein substance) increase. As an answer to the overloading of the myocardium, many authors have established an intensification in the synthesis of myocardial proteins. It is, however, little known which proteins are then more intensively created and accumulated. This study examines the dynamics of the protein and nonprotein content as well as of single protein fractions (sarcoplasmic, myofibrillar, and stromal) in both hypertrophied and normal tissue from rat myocardia. The results revealed that in Goldblatt rats, 4-24 weeks after stenosis of one renal artery, no noteworthy differences in the relationships of protein and nonprotein content were caused by hypertrophy (34-54%) due to left ventricular pressure overload. The same is true of the tissue from moderately hypertrophied myocardia (12-17%) of rats exercised for several weeks by swimming training. Determination of hydroxyproline concentration showed that significant differences in the content of the collagen tissue in relation to control animals of the same age occurred only in Goldblatt rats 24 weels after operation. However, greater alterations in the concentrations of various protein fractions could be registered. The increase in the concentration of myofibrillar proteins in hypertrophied myocardial tissue is of particular significance and is to be considered as an adaption of the muscle to the increased mechanical demands. Certain changes regarding the relation of the single components within the myofibrillar fraction (relation of actomyosin concentration to T-fraction; relation of both components to total fraction), whose cause and significance is as yet unclear, could be observed.