Oxytocin (OT) and its receptor (OTR) are synthesized in the endometrium and myometrium of the pregnant rat during late gestation. Both are regulated by estrogen and progesterone (P4), and tissue concentrations of both increase markedly before parturition. The P4 antagonist RU486 will induce parturition in the rat. The purpose of the present studies was to investigate changes in OT and OTR messenger RNA (mRNA) and peptide synthesis within the pregnant rat uterus during RU486-induced parturition. Pregnant rats were given a single injection of RU486 (2.5 mg/rat in oil) on day 15 of pregnancy (normal delivery occurs on day 22). Control animals received injections of oil only. Groups of animals (n = 5 in each group) were euthanized at 0, 6, 12, 24, and 48 h after injection and during labor (immediately after delivery of the first pup). Maternal serum estradiol (E2), P4 and uterine OT, and PGE2 concentrations were measured by RIA. Prostaglandin F2alpha and estrogen receptor levels were measured by enzyme immunoassay (EIA). OTR and P4 receptor (PR) were measured using radioligand-binding assays. OT, OTR, and estrogen receptor mRNAs were measured with ribonuclease protection assays. The average time to delivery, after RU486 injection, was 27.0 +/- 1.2 h. Serum E2 and P4 levels were increased slightly, but significantly, at 24 h after RU486. In controls, OT mRNA increased significantly, and this increase was blocked in the RU486 treatment group. OTR mRNA levels increased within 6 h of RU486 and remained elevated until delivery. OTR peptide was increased by 12 h. PGE2 and PGF2alpha were increased 3-fold and 16-fold, respectively, but not until after the increase in OTR had occurred. We conclude that the mechanism of action of RU486 is to inhibit the P4 suppression of OTR synthesis, allowing increased expression of OTR, which may directly stimulate myometrial contractions or act indirectly through increased synthesis of PGs.