A new laryngeal mask airway, the ProSeal (PLMA), is said to be more difficult to insert than the laryngeal mask airway Classic (CLMA) using propofol anaesthesia. Therefore, we expected a greater dose of propofol and sevoflurane to be required to insert the PLMA compared with the CLMA. We determined the effective concentration 50% (EC(50)) of propofol and end-tidal sevoflurane to allow insertion of the PLMA and the CLMA. Seventy-six elective female patients (aged 20-60 yr and ASA I-II) were randomly assigned to one of four groups. Either a PLMA or a CLMA was inserted using either propofol target controlled infusion or sevoflurane. Both propofol and sevoflurane targets were determined with a modified Dixon's up-and-down method. After equilibration between the predetermined blood and effect site concentrations, which had been held steady for more than 10 min, LMA insertion was attempted without neuromuscular block. The predicted EC(50CLMA) and EC(50PLMA) for propofol were 3.14 (0.33) and 4.32 (0.67) micro g ml(-1). E'(CLMA) and E'(PLMA) of sevoflurane (mean (SD)) were 2.36 (0.22) and 2.82 (0.45)% (P<0.01 and 0.05, respectively). The estimated concentration of propofol and the sevoflurane concentration needed to allow insertion of the ProSeal are respectively 38 and 20% greater than those needed for insertion of the Classic LMA.