Progesterone retards postpartum involution of the rabbit myometrium.

Postpartum involution of the uterus was significantly reduced in rabbits by immediate and repeated injections of progesterone. Progesterone had no effect if the injections commenced 24 hours after delivery. The decrease in concentration of progesterone that accompanies birth results in release of proteolytic and other degradative enzymes from subcellular particles; these enzymes cause the involution, and, once the enzymes are released into the cellular milieu, progesterone has no effect.