Abstract: In order to investigate the kinetic changes of endogenous prostaglandin (PG) concentrations during the process of abortion induced by intra-amniotic injection of rivanoi, serial samples of amniotic fluid were taken before and after rivanol administration in 10 mid-term pregnant women. The PG concentrations were determined by radioimmunoassay. Amniotic fluid PGE increased from a pre-rivanol level Of 213±54 to 553±138 (pg/ml), ten to fifteen hours after administration, 968±243 (pg/ml) at commencement of uterine contraction and 1475±869 (pg/ml) before abortion with cervix diameter of 3～4 cm; the corresponding values for PGF_2α were 369±104, 392±167, 1582±548 and 6.006±2131 (pg/ml).The results indicate that the PGE and PGF_2α levels in the amniotic fluid increased gradually in the course of rivanol-induced abortion, and the increase of PGE level was initiated long before the commencement of urerine contraction. It is, therefore, concluded that the increase of PG level in the amniotic fluid may be responsible for the initiation and progress of rivanol-induced abortion.