The dose-response and concentration-response relation of rocuronium infusion was studied in 20 adult surgical patients during propofol-nitrous oxide and isoflurane (1 MAC)-nitrous oxide anaesthesia. Neuromuscular block was kept constant, initially at 90% and then at 50% with a closed-loop feedback controller. At 90% block the steady-state infusion of rocuronium was 0.55 +/- 0.16 mg kg-1 h-1 and the corresponding concentration 1714 +/- 281 ng mL-1 in patients receiving propofol. At 50% block the corresponding infusion rate was 0.27 +/- 0.11 mg kg-1 h-1 and the concentration 1077 +/- 244 ng mL-1, respectively. At 50% block isoflurane reduced the rate of infusion by 52% (P < 0.005) and the concentration by 59% (P < 0.001); at 90% block both the mean infusion rate and the concentration of rocuronium were reduced by 35% (P < 0.005). The mean rocuronium clearance at 50% block was unaffected by the type of anaesthesia; it was 4.1 +/- 1.6 and 4.9 +/- 2.7 mL kg-1 min-1 in the groups receiving propofol and isoflurane anaesthesia, respectively. We conclude that isoflurane reduces the infusion requirements of rocuronium by changing the pharmacodynamic behaviour.