Thirty patients requiring elective ventilation in the ICU received either intermittent boluses (25 patients) or a continuous infusion (five patients) of rocuronium. Degree of block was monitored by train-of-four stimuli and maintained at one twitch either observed or palpated. All patients were sedated with intermittent doses or an infusion of 0.14-0.4 microgram kg-1 h-1 of midazolam and received a continuous infusion of either sufentanil or fentanyl (0.25-2.0 micrograms kg-1 h-1). Control of ventilation was better with the continuous infusion of rocuronium, but these patients also had a more intense block receiving 9.9 +/- 1.3 micrograms kg-1 min-1 as compared to 6.4 +/- 2.3 micrograms kg-1 min-1 in the bolus group. Elimination half-time, volume of distribution at steady-state, and mean residence time were significantly greater than in surgical patients receiving comparable infusions, but plasma clearance was similar.