['Magnesium sulfate, mainly used in obstetrics to treat eclamptic convulsions, is currently questioned as to its clinical tocolytic effect. We aimed to study the relaxant action (if any) of magnesium sulfate on in vitro pregnant and non-pregnant myometrium.', 'Myometrial strips, harvested from five pregnant women (35-39 gestational weeks) during Cesarean procedures indicated for dystocia or scared uterus and five non-pregnant women during hysterectomy or myomectomy for benign conditions, were placed in a Krebs-Henseleit solution organ bath and the isometric force was registered. We assessed the effect of Mg2+ (magnesium sulfate) at different concentrations (0.50-10 mM) on spontaneous and oxytocin-induced (1 microM) myometrial contractility.', 'Mg2+ temporarily reduced spontaneous myometrial contractions in a dose-dependent manner, with efficient regimens at 2.0-2.5 mM, and arrested contractility completely at 3 mM. Oxytocin-induced contractions were reduced by 30-40% at 8 mM and decreased further at 9-10 mM. Induced contractions were reduced, in a dose-dependent and time-dependent manner (maximum effect at 20 min), at higher Mg2+ concentrations and with non-significant proportional differences between pregnant and non-pregnant myometrium.'] Does magnesium ion inhibit spontaneous and induced contractions of isolated uterine muscle?