A technique is described for studying endometrial or decidual blood flow by introducing a thermistor, mounted in a flexible catheter, into the uterus. The technique was first tested in model experiments and in the pregnant rabbit and was then used to study the blood flow of the human uterus. In pregnant women, the thermistor was introduced between the decidua and fetal membranes, and in non-pregnant women, it was applied to the endometrium of the fundus. Intra-uterine pressure was recorded simultaneously. No complications were encountered in either pregnant or non-pregnant patients. Decidual or endometrial blood flow remained steady over long periods. There were transient fluctuations about the mean level, usually associated with myometrial contractions, but these could easily be distinguished from changes in the level of blood flow evoked by administration of vasoactive drugs.