Atosiban acetate (RW22164) is an inhibitor of the hormones oxytocin and vasopressin. It is used as an intravenous medication as a labour repressant (tocolytic) to halt premature labor. Atosiban acetate (RW22164) inhibits the oxytocin-mediated release of inositol trisphosphate from the myometrial cell membrane. As a result, there is reduced release of intracellular, stored calcium from the sarcoplasmic reticulum of myometrial cells, and reduced influx of Ca2+ from the extracellular space through voltage gated channels. In addition, Atosiban acetate (RW22164) suppresses oxytocin-mediated release of PGE and PGF from the decidua. In human pre-term labor, atosiban, at the recommended dosage, antagonizes uterine contractions and induces uterine quiescence. The onset of uterus relaxation following atosiban is rapid, uterine contractions being significantly reduced within 10 minutes to achieve stable uterine quiescence.