Twenty patients in the intensive care unit received an infusion of atracurium to permit mechanical ventilation. The duration of infusion ranged from 38 to 219 h and the average rate of infusion during the study was 0.76 mg kg-1 h-1. In 14 patients an increase in atracurium requirement occurred within the first 72 h of the infusion. Recovery from neuromuscular blockade after a prolonged infusion was sufficiently rapid to avoid pharmacologically induced reversal. In six patients maximum plasma concentrations of laudanosine were 1.9-5 micrograms ml-1, and there was no evidence of cerebral excitation.