SummaryWe assessed the effect of magnesium on the amount of bleeding, coagulation profiles and surgical conditions during lumbar discectomy under general anaesthesia. Forty patients, of ASA physical status 1-2 and aged 18-65 years, undergoing single-level microscopic lumbar discectomy, were randomly assigned to magnesium sulphate (50 mg.kg )1 in 100 ml saline over 10 min followed by a continuous infusion of 20 mg.kg.h )1 ) or saline. The mean (SD) estimated blood loss was 190 (95) and 362 (170) ml in the magnesium and saline groups, respectively (mean difference = 172 ml; 95% CI 84-260 ml). The median (IQR [range]) Fromme's scale score for surgical conditions for the magnesium and saline groups were 2 (2-3 [2-3]) and 3 (2-3 [3-4]), respectively (p < 0.05).The bleeding time, haemoglobin, platelet count, prothrombin time, international normalised ratio and fibrinogen levels were similar in the two groups. The activated partial thromboplastin time was prolonged in the magnesium group immediately postoperatively and at 6 h after surgery. After the bolus of magnesium, the heart rate was higher and the mean arterial pressure lower in the magnesium group. The use of magnesium sulphate during lumbar discectomy decreases blood loss, and provides better surgical conditions without marked haemodynamic effects.