Preparations of fragmented sarco plasmic reticulum (SR) from bovine uterus were found to be considerably more abundant when originating from pregnant than from non-pregnant cows ATP-dependent calcium binding nant cows. was not significantly different in uterine SR from pregnant and non-pregnant cows. Because of the greater yield, the total calcium binding capacity was significantly increased in SR from pregnant uterus. ATP-dependent calcium binding of the SR preparations was completely inhibited by 3 mM salyrgan, but not by dicumarol. Approximately 25% of the intrinsic calcium was exchangeable with 45Ca. The amount of calcium bound to the sarcoplasmic reticulum suggests that it plays a significant role in myometrial contraction.