In animal studies, ondansetron, 5-hydroxytryptamine type-3 receptor antagonist, increases the effect of neuromuscular blockade of rocuronium, which associated with structural similarity of 5-hydroxytryptamine type-3 receptor to nicotinic acetylcholine receptor. We have examined whether ondansetron, which is often used to prevent nausea and vomiting after surgery, affects the onset time of neuromuscular blockade of rocuronium. A total of 52 adult patients were randomly allocated into either the saline group (N=27) or ondansetron group (N=25) for general anesthesia. The ondansetron group received 8 mg of ondansetron intravenously and the control group received 4 ml of normal saline intravenously 3 minutes before induction of anesthesia. Neuromuscular function was assessed by acceleromyography of the adductor pollicis with a train-of-four stimulation. The onset time was defined as the time in seconds from the start of injection of rocuronium until 0 of the train-of-four count. The onset time of rocuronium in the ondansetron group (145.4 ± 56.7, 95% CI: 122 to 169 sec) was not significantly different compared to that of the control group (133.0 ± 46.4, 95% CI: 115 to 151 sec) (P=0.394). Pretreatment of ondansetron before anesthesia induction did not affect the onset time of rocuronium induced neuromuscular blockade.