The effects of sufentanil 0.5 or 1 microgram/kg, given intravenously after induction of anaesthesia, on the cardiovascular responses to tracheal intubation were examined in a controlled, randomised, double-blind investigation. The control group of patients exhibited significant rises in arterial blood pressure and heart rate for 4 minutes after tracheal intubation. Heart rate exceeded 100 beats/minute and systolic pressure increased by over 20% in every patient. All patients moved or breathed within 10 minutes of the administration of suxamethonium. Sufentanil 0.5 microgram/kg prevented increases in the mean values of heart rate and arterial blood pressure, although increases were observed in five patients. Significant falls in the mean values of heart rate and arterial pressure occurred from 4 minutes after intubation until observations ended 15 minutes after induction of anaesthesia. Two patients moved or breathed during this time, although movement in response to nerve stimulation occurred in all patients 10 minutes after administration of suxamethonium. Sufentanil 1 microgram/kg was effective in suppressing a rise in heart rate or arterial pressure in every patient. Significant falls in these variables occurred from 2 minutes after tracheal intubation onwards. No patient moved or breathed for 15 minutes after induction of anaesthesia, although neuromuscular transmission was present 10 minutes after giving suxamethonium in each case.