Induced water diuresis blocks the central release of antidiuretic hormone (ADH) which is, in turn, supposed to play a role in the high uterine activity recorded during human menstruation. In order to further explore this possibility, uterine and antidiuretic activities were studied during water diuresis in 27 normal menstruating women, prior to the insertion of an intrauterine device. Shortly after the initiation of the water overload, simultaneous inhibition of both uterine and antidiuretic activities was obtained. Furthermore, the administration of ADH by intravenous infusion re-established the uterine motility to normal values while the administration of oxytocin did not. It is suggested that ADH has a direct role in the control of uterine activity during human menstruation.