I T WAS long suspected, before actual proof became available, that the corpus luteum was an inhibitor of uterine contractions. This suspicion arose from the observat,ion that suppression of uterine contract,ions paralleled the life of the corpus 1uteum.l Since 1929 there has become available potent extracts of the corpus luteum, and more recently, the synthetic progesteronr.2 These discoveries were followed by a mass of animal experimentation. An exhaustive monograph describing the early changes in the uterus of t.he pregnant rabbit was published by Knaus.” The livid, flaccid uterus associated with pregnancy and the development of the corpus luteum were graphically described. That this flaccidity was the result of corpus lutenm influence and not the pregnancy directly was proved by experiments with sterile coitus in the rabbit. Sterile coitus was shown to induce follicle rupture and corpus luteum development which was followed by the same uterine relaxation as that seen in pregnancy. A number of investigators have confirmed t,his observation.” This has been described as the reaction of pseudopregnancy. By means of “in viva” studies, using for the most part the artificially produced utcroabdominal fistula, the effect of corpus luteum extracts and synthetic progesterone have been studied.5 These observations have been made on the rabbit and confirmed directly or indirectly by experiments on the cat, cow, and sow. Experiments on excised ut,erine muscle have been unsatisfactory because of the insolubility of the hormone. There is abundant evidence which no one could deny that progesterone inhibits uterine contractions in certain animals. Several years ago Falls published the results of certain experiments on the human being, using the aqueous extract of corpus luteum.6 An intrauterine balloon c.onnected t,o a mercury “U” tube wit,h a writing point on one arm was used to record the contractions on a kymograph. Inhibition of uterine contractions within an amazingly short time was recorded. I have not been able to confirm this work. In fact, I have not been able to demonstrate any inhibitory action of progesterone on uterine contractions. It was to be expected that such a report, however, would be enthusiastically received by the profession for the treatment of conditions wherein there appears to be a hypercontractility of the myometrium. The results of extensive animal esperimentation and nnconfirmed work on the human being inspired a premature conclusion