The effect of fentanyl, 8 micrograms/kg, used as an adjunct to thiopental for induction of anesthesia, on the circulatory response to tracheal intubation was investigated in 36 patients undergoing major vascular surgery. Patients were randomly assigned to receive either thiopental, 6 mg/kg, alone (N = 18), or thiopental, 3 mg/kg, along with fentanyl, 8 micrograms/kg (N = 18), for induction of anesthesia. The electrocardiogram, arterial pressure, pulmonary capillary wedge pressure, cardiac output, and central venous pressure were measured during induction of anesthesia, laryngoscopy, and intubation. Mean arterial blood pressure increased more following intubation in patients given thiopental than in patients given fentanyl-thiopental, reaching a peak value of 144 +/- 4 torr in patients receiving thiopental only, compared with 108 +/- 6 torr in those receiving fentanyl and thiopental (p less than 0.0001). Increases in systolic blood pressure, diastolic blood pressure, and pulmonary capillary wedge pressure with intubation were also significantly greater following administration of thiopental than following fentanyl-thiopental. Doses of fentanyl that are low enough to cause little postoperative respiratory depression significantly blunt postintubation hypertension when used as an adjunct to thiopental.