The safety and efficacy of atracurium 0.8 mg kg-1 was determined in healthy patients with particular attention to the speed of onset of blockade, and to changes in haemodynamic variables. Atracurium 0.8 mg kg-1 had a shorter onset time than atracurium 0.5 mg kg-1, and satisfactory intubating conditions were achieved earlier. "Priming" produced no significant improvement in onset time or intubating conditions. Onset times were significantly shorter with nitrous oxide-opioid anaesthesia than following thiopentone alone. Although a 0.8-mg kg-1 bolus resulted in a significant reduction in mean arterial pressure to 75% of control and was associated with a significant increase in plasma histamine concentrations, this response could be prevented by injecting the drug over 75 s. "Priming" or a 30-s injection produced no haemodynamic protection. The protection achieved by pretreatment with anti-histamines was incomplete: mean arterial pressure decreased to 83% of control.