OBJECTIVE To determine effects of castor oil-diet on the synthesis of prostaglandin E2(PGE2) and explore the mechanism of labor induced by castor oil-diet in pregnant rats. METHODS Pregnant rats were gavaged castor oil-diet in 18 and 19 days of gestation. At the time of death, blood of portal and peripheral veins, intestinal mucosa, amnion, amniotic cells and placenta were collected, and the tissues were cultured in the presence of ricinoleic acid or indomethacin. The concentrations of PGE2 in the media or blood were measured by RIA methods. RESULTS The PGE2 levels in the portal vein increased, while the PGE2 levels in peripheral blood had no changes significantly, the PGE2 levels in the tissues of the intestinal mucosa, placenta, amnion and amniotic cells were increased significantly; Ricinoleic acid stimulated the synthesis of PGE2 in the above tissues in vitro, which had the positive correlations with the dose of ricinoleic acid and its lasting time. Indomethacin inhibited the synthesis of PGE2 in-vitro. CONCLUSIONS The increased synthesis of PGE2 in the intrauterine tissues is a key of the initiation of labor induced by castor oil-diet, and ricinoleic acid in castor oil-diet might be the active component which induced the initiator of labor.