Purified preparations of fragmented sarcoplasmic reticulum (SR) from pregnant bovine uterus were allowed to bind calcium in a medium containing ATP, Mg++ and Ca++. The smooth muscle stimulating prostaglandins (PG) E2 and F2Α inhibited calcium binding. The physiologically inactive PGF1Β had no effect. Similarly, addition of PGE2 and PGF2Α to the incubation medium after calcium binding was complete, led to release of some of the bound calcium, depending on the PG concentration. Intrinsic, membrane-bound calcium could be partially removed with either EGTA (1 mM) or PGE2 (100 µg/ml) but not with PGF1Β. The amount of intrinsic calcium removed was greater after EGTA than after PGE2 treatment. Following removal of calcium with EGTA or PGE2, renewed binding of calcium took place in the absence and, more so, in the presence of ATP. However, the initial level of intrinsic calcium was not completely restored. EGTA treatment of SR preparations would also remove calcium following ATP-dependent calcium binding. Renewed incubation restored ATP-dependent calcium binding. These observations suggest that the interaction of PGs with calcium may be the basis for the increased contractile force of the uterus observed in labor.