A 4-week toxicity study of magnesium sulfate administered by 24-hr intravenous infusion at the dosage levels of 0, 12.5, 50 and 100 mg/kg/hr in female beagle dogs was conducted. No death occurred in any group. Changes attributable to the treatment with magnesium sulfate were decreased food consumption and body weight gain, anemic change, increased urine volume, decreased serum calcium level, increased inorganic phosphorus level, slight prolongation of conduction time in electrocardiogram and tubular basophilia in the kidneys in the group treated with 100 mg/kg/hr. In addition, essentially similar changes were also observed at the same dosage level in the 2-week study of this drug, in which recoverability was recognized with 2-week follow-up observation after drug withdrawal. In conclusion, the nontoxic dosage level was judged to be 50 mg/kg/hr under the condition of the present study.