BACKGROUND: A previously published study suggested that pre-treatment with magnesium sulphate (MgSO(4)) had no impact on the speed of onset of rocuronium-induced neuromuscular block. We set out to verify this assumption. METHODS: Eighty patients (18-60 years) were randomly allocated to MgSO(4) 60 mg/kg or placebo (saline). Study drugs were given intravenously for 15 min before induction of anaesthesia with propofol, sufentanil and rocuronium 0.6 mg/kg. Anaesthesia was maintained with a target-controlled propofol infusion. Neuromuscular transmission was measured using train-of-four (TOF)-Watch SX acceleromyography. RESULTS: Onset was analysed in 37 MgSO(4) and 38 saline patients, and recovery in 35 MgSO(4) and 37 saline patients. Onset time (to 95% depression of T1) was on average 77 [SD=18] s with MgSO(4) and 120 [48] s with saline (P<0.001). The total recovery time (DurTOF0.9) was on average 73.2 [22] min with MgSO(4) and 57.8 [14.2] min with saline (P<0.003). The clinical duration (Dur25%) was on average 44.7 [14] min with MgSO(4) and 33.2 [8.1] min with saline (P<0.0002). The recovery index (Dur25-75%) was on average 14.0 [6] min with MgSO(4) and 11.2 [5.2] min with saline (P<0.02). The recovery time (Dur25%TOF0.9) was on average 28.5 [11.7] min with MgSO(4) and 24.7 [8.4] min with saline (P=0.28). CONCLUSION: Magnesium sulphate given 15 min before propofol anaesthesia reduces the onset time of rocuronium by about 35% and prolongs the total recovery time by about 25%.