Effects of calcium-free perfusion and calcium-free perfusion followed by reperfusion with calcium on sarcolemmal structure, sarcolemmal permeability, and creatine phosphokinase loss were investigated in isolated perfused rat hearts. Release of creatine phosphokinase was significant (P less than 0.0002) after 4-5 minutes of perfusion with Ca++-free medium, but later releases in comparison to their immediately preceding periods became significant only after more than 20-minute perfusion. Poor correlation between enzyme loss and lanthanum permeability prior to 20 minutes of Ca++-free perfusion was noted. After 20 minutes of Ca++-free perfusion, the basal lamina was separated from the plasma membrane, and lanthanum was seen in the cytoplasm. The intramembranous particles began to aggregate at that time. The morphologic and enzymatic changes were dramatic following reperfusion of calcium-free perfused hearts. Morphologic changes in these hearts included separation of basal lamina, cellular separation at the intercalated disk, dissolution of actin filaments at the region of I band, contraction bands, cell swelling, and staining or filling of mitochondrial membranes with La+++. Increased sarcolemmal permeability was associated with tears and aggregation of intramembranous particles in the sarcolemmal lipid bilayers. These results suggest that reperfusion of Ca++-free perfused cells causes irreversible damage to the sarcolemmal lipid bilayer, and the degree of alterations induced in the cells is dependent upon the initial duration of Ca++-free perfusion.