Many techniques, especially the intra-uterine rubber balloon, have been used for the recording of uterine contractions in the pregnant and non-pregnant woman. The size of the balloon and its tension, however, greatly influence the results, as Reynolds (1949) has pointed out. Furthermore, it is difficult to maintain exactly the same balloon volume throughout several hours' recording.Some years ago Hendricks (1964) introduced the`o pen end' catheter technique which is rather simple: a thin, fluid-filled polyethylene catheter is introduced into the cavity of the non-pregnant uterus, and its outer end is connected to a recording system. The fluid surface at the inner tip of the catheter acts as a pressure membrane which transmits the pressure in the slit between the anterior and posterior uterine wall to the fluid within the catheter and thence to the recording instrument. In trained hands this technique works very well and it has been successfully used by