45 neurologically healthy patients undergoing continuous spinal anaesthesia for hip arthroplasty were randomized to receive intravenous magnesium sulphate (MgSO4) or saline. Over 6 hours, the magnesium concentration in the serum increased in the MgSO4 group and decreased in the saline group. However, both groups demonstrated a decrease in the cerebrospinal fluid (CSF) magnesium concentration at 2 hours. This concentration returned to baseline by 6 hours. There was no difference between the two groups at any time period for the concentration of CSF magnesium or for any other ion concentration in the CSF.