Purified preparations of fragmented sarcoplasmic reticulum from non-pregnant bovine uterus bind calcium from a medium containing ATP and Mg++. This calcium binding was enhanced in the presence of progesterone, while it is decreased in the presence of oxytocin. In the presence of both hormones there was an intermediate effect. PGF2Α, when added to preparations that had previously bound calcium in the presence of progesterone, caused release of limited amounts of calcium. Under these conditions calcium binding was much higher than that observed in the presence of PGF2Α alone. Three steroid analogs showed no effect on ATP-dependent calcium binding.