Abortifacient effects of 16,16-dimethyl-trans delta 2-PGE1 methyl ester (ONO-802) were studied clinically. The uterine contractile effect of the agent was compared with those of PGF2 alpha and oxytocin (OXY) in the unanesthetized rabbit. 1. Intermittent intravaginal administration of ONO-802 was applied to 32 cases of legal abortion, 15 of missed abortion and 17 of hydatid mole. Eighty eight, 100 and 81 per cent of these patients resulted in abortion, respectively, with fewer side effects than those of natural PGs. 2. In the five groups of non- or pseudo-pregnant rabbits and those in their 7-9, 14-16 and 19-28 days in pregnancy, uterine contractile effects of these agents were assessed by both the contractile patterns and area of contractile curves of initial 5 minutes. The results are as follows: 1) In the non-pregnant rabbits, all of these agents revealed marked uterine contractile effect. 2) ONO-802 induced uterine contraction characterized by its wedge-shaped curves continued considerably longer than that induced by others. 3) ONO-802 revealed much stronger effect on uterine contraction in 7-9 day-of-pregnant rabbits. 4) Fourteen-16-day-of-pregnant rabbits were least influenced by the three agents as regards their uterine contraction in accordance with the highest progesterone levels in their sera among the three groups of pregnant rabbits.