Objectives: The purpose of this study was to determine the effect of magnesium sulfate on protein kinase C (PKC) translocation in myometrial cells from pregnant women. Study design: Myometrium was obtained at the time of cesarean delivery from women at term before labor. Cultured myometrial cells were treated with magnesium sulfate (3, 5. and 10 mmol/L). oxytocin (0.1 μmol/L), or 12-O-tetradecanoylphorbol-13-acetate (TPA, 0.1 μmol/L). The translocation of PKC isozymes alpha (calcium dependent) and delta (calcium independent) from cytosol to membrane fractions was assessed with use of Western blot analysis. Results: In unexposed control cells, the majority of PKC alpha and delta was located in the cytosol fraction. Exposure to magnesium sulfate for 60 minutes induced translocation of both PKC alpha and delta at concentrations as low as 5 and 3 mmol/L. respectively. The magnitude of magnesium sulfate induced translocation for PKC delta is similar to that seen after oxytocin or TPA exposure but less for PKC alpha. Exposure to oxytocin for 30 minutes and 60 minutes induced translocation of PKC alpha and delta, respectively. Exposure to TPA for 5 and 30 minutes induced translocation of PKC alpha and PKC delta, respectively. In calcium-free media, only TPA induced translocation of these two isoenzymes. Conclusion: Magnesium sulfate stimulates PKC translocation in cultured myometrial cells from pregnant women. Magnesium sulfate and oxytocin require extracellular calcium to induce translocation of both PKC alpha and delta. © 2004 Elsevier Inc. All rights reserved.