Heart myocytes subjected to ischemia show a clustering of the sarcolemmal proteins. In the erythrocyte membrane, a system in which intramembranous particle (IMP) aggregation is extensively studied, it is found that an IMP aggregation can in principle only occur upon removal of the membrane skeleton of spectrin and actin by rather drastic experimental conditions. With regard to phospholipid composition and topology the sarcolemma and the erythrocyte membrane show large similarities and therefore it was proposed that a loss of the interaction of the IMPs and the cytoskeleton is also a prerequisite for the sarcolemmal IMP aggregation (Verkleij et al., 1990). Freezing myocardial tissue, both from adult and neonatal rat, from temperatures lower than 37 degrees C resulted in an aggregation of the sarcolemmal IMPs. The aggregation is proportional to the degree of lowering of the temperature at which the tissue is cryofixed. This in contrast to the erythrocyte membrane, where lowering the temperature only induces moderate IMP aggregation. The IMP aggregation in the sarcolemma is reversible upon a subsequent increase in incubation temperature. The results clearly demonstrate that the interaction between the sarcolemmal proteins does not hinder aggregation of the IMPs, as proposed previously, and suggest that loosening of this complex does not have to proceed the aggregation of the sarcolemmal intramembranous particles during ischemia.