The duration of action of atracurium was studied in two groups of patients. In group I (n = 15), patients' lungs were hyperventilated to a mean PaCO2 of 3.3 kPa and in group II (n = 15) lungs were ventilated to maintain a mean PaCO2 of 5.3 kPa. Anaesthesia was maintained using an infusion of propofol. The time taken to return to a T1 twitch height of 10% of control was measured following each incremental dose of atracurium 0.15 mg kg-1. It was found that the duration of action of atracurium was significantly (P less than 0.0005) shorter in the hypocapnic group.