The effect of intravenous infusion of the pure prostaglandin compounds PGE1 and PGE2 on the motility of the pregnant human uterus was studied. Recording of uterine motility was made by measuring the amniotic pressure. Nine midpregnant and 8 term pregnant patients were studied. Both PGE1 and PGE2 stimulated uterine motility in all experiments. The potency of PGE1 was approximately the same as that of PGE2. The midpregnant uterus was somewhat more sensitive to both compounds than the uterus at term. Threshold dose at midpregnancy was 2 to 4 μg per minute and at term 4 to 8 μg per minute. At midpregnancy the PGE compounds preferably induced an increment of tone but to some extent also an irregular uterine activity. At term the effect on amplitude and frequency of contractions was more dominant but unphysiologic elevation of tone frequently occurred. The latter reaction renders the PGE substances less suitable for induction of labor, whereas their tendency to cause longlasting uterine contracture might offer a group of new pharmacologic agents for the treatment of uterine atonicity after delivery.