Sugammadex, a chemically modified gamma-cyclodextrin, was synthesized as a selective reversal agent for rocuronium, a steroidal neuromuscular blocking agent. The lipophilic cavity of sugammadex molecule encapsulates the lipophilic steroid rings of rocuronium and forms 1:1 host-guest complex. Additionally, negatively charged carboxyl groups around the rim of cyclic sugammadex easily attract a positively charged quaternary nitrogen of rocuronium. When sugammadex 8 mg x kg(-1) was given intravenously during a profound block induced by rocuronium, a train-of-four ratio of the thenar muscle to the ulnar nerve stimulation returned to 0.9 within 2 min. The rapid reversal can be explained by the rapid redistribution of free rocuronium from the neuromuscular junction to the plasma. The reversal effects are not influenced by anesthetics. There is no interaction with cholinergic systems. Therefore, undesired side effects attendant with anticholinesterases are not seen. No recurarization was also observed. The majority of the complexes is excreted via the kidney, but the reversal effects are complete even in patients with renal failure.