Smooth muscle microsomal vesicles were loaded with calcium in the presence of oxalate and ATP. The intact vesicles that contained calcium oxalate crystals were separated by ultracentrifugation from empty vesicles. This results in a unique model system, composed of a biologically active membrane with virtually all of the calcium inside. Intravesicular calcium is differentiated from externally bound calcium using ionophore X537A and EGTA. EGTA released calcium slowly with a half time of 93 min. The ionophore X537A rapidly released calcium with a half time of 12 min. This model can be used to test for calcium ionophoretic action. Prostaglandin (PG) E2 and prostacyclin (PGI2) were tested in this system. We found that PGE2 and PGI2 did not change calcium permeability.