Myometrium from rats in varying stages of pregnancy (from 17 to 22 days) was treated with oxytocin (0.1-10 microM) and plasma membranes and sarcoplasmic reticulum were isolated using a Percoll gradient. When the myometrium had been treated with oxytocin, Ca2+ uptake was reduced by 29.4% in the plasma membrane and by 32.6% in the sarcoplasmic reticulum. The inhibitory action of oxytocin was highly dependent upon the stage of gestation: Only membranes from rats close to term (21-22 days) exhibited reduced Ca2+ transport activity after hormone treatment. This effect correlated highly with a significant decrease in the serum progesterone level of these animals. In plasma membrane vesicles, oxytocin reduced the maximal velocity of the Ca2+ pump without significantly affecting the affinity for Ca2+. Oxytocin did not affect the passive permeability of the plasma membranes, nor their proportion of sealed inside-out vesicles nor the amount of Ca2(+)-pump protein in these membranes. In addition, oxytocin caused no change in the passive permeability of the membrane nor in the rate of inositol trisphosphate-induced Ca2+ release from the sarcoplasmic reticulum. These results suggest that there is a specific action of oxytocin on the activity of the myometrial plasma membrane and sarcoplasmic reticulum Ca2+ pumps which may contribute to the maintenance of an elevated intracellular calcium level during parturition.