Administration of ephedrine prior to rocuronium decreases the onset time of neuromuscular blockade from rocuronium by 26%. This effect was attributed to a increased cardiac output. If so, β adrenergic-blocking drugs, which decrease cardiac output, should prolong the onset time of rocuronium. In a double-blind study, 60 patients were randomly assigned to three groups (n = 20) to receive either 70 μg · kg-1 of ephedrine, 0.5 mg · kg-1 esmolol or placebo, 30 s before induction of anesthesia. Onset time of rocuronium was defined as the time from the end of its injection to disappearance of all four twitches of the train-of-four. The onset time of rocuronium was significantly shorter after ephedrine (22%) and longer after esmolol (26%), as compared to placebo. No differences were observed among the three groups with regard to heart rate, systolic, diastolic or mean blood pressure. We concluded that a dose of 0.5 mg · kg-1 of esmolol significantly prolongs the onset time of rocuronium with minimal hemodynamic changes.