Studies on the in vitro interaction of β-endorphin with different biomembrane preparations, measured in terms of Ca2+ induced chlorotetracycline (CTC) fluorescence alterations indicate that β-endorphin causes release of calcium ion from both synaptosomal and erythrocyte membrane stores but not from rat brain mitochondria. It seems that the binding pattern or interaction of β-endorphin with synaptosomes and erythrocytes are characteristically different from that of mitochondria, indicating some functional significance of this differential interaction. © 1983.