Ca2& taken up by sarcoplasmic reticulum membrane fragments (SRF) upon using ATP could be released rapidly by changing the anion outside the vesicles from methanesulfonate to chloride. It is considered that this anion exchange caused depolarization of the sarcoplasmic reticulum membrane. Similar rapid release of Ca2& taken up by SRF was also caused by a change from high to low osmotic pressure, probably due to bursting of the membrane. On the basis of experiments in which these two types of Ca2& release were discriminated, it was concluded that Ca2& bound inside the membrane was released directly by anion exchange (depolarization). However, Ca2& release was not caused by cation exchange. Sucrose inhibited these two types of Ca2& release. Cia2& taken up in the presence of oxalate could not be released by any treatment used. Liver microsome fraction also has Ca2& uptake activity. However, Ca2& was not released upon anion exchange, but was released upon oxmotic change. These results show that Ca2& release from SRF upon anion exchange is specific to the sarcoplasmic reticulum membrane. In conclusion, SRF membrane retains the ability to respond to the depolarization caused by ion exchange and can release the accumulated Ca2&.