The effect of oxytocin infusion on the pharmacokinetics of standard intramuscular magnesium sulfate therapy was determined in 18 women with preeclampsia; the results were compared with those in seven women with preeclampsia who did not receive oxytocin. Oxytocin had no significant effects on the maternal serum magnesium and calcium ion concentrations, nor did oxytocin appear to affect the magnesium or calcium concentrations in fetal_ umbilical cord blood. Urinary excretion of magnesium rose 21-fold and calcium excretion rose threefold in patients receiving intramuscular magnesium sulfate in both the oxytocin and the nonoxytocin groups. Sixty-five percent of the administered magnesium was excreted during the treatment period, again with no significant differences between the oxytocin and the nonoxytocin groups. These results indicate, that oxytocin does not affect the pharmacokinetics of intramuscular magnesium sulfate and no dosage adjustment of magnesium sulfate is required when oxytocin is used to induce or augment labor or when it is given during the postpartum period. © 1987, C. V. Mosby Company. All rights reserved.