We present a case where rocuronium 80 mg (3 x ED95) was used in a rapid sequence induction in a 80-kg pregnant patient with an open eye injury. The patient was also receiving magnesium 2 g h-1 i.v. for preterm labour. The expected duration for neuromuscular block of rocuronium in the absence of magnesium would be approximately 53 min; with infusion of magnesium, the duration of neuromuscular block was prolonged four-fold (215 min). It is important to remember that magnesium potentiates the effects of all non-depolarizing neuromuscular blocking agents, including rocuronium.