To define the role of muscle relaxants in hemodynamic responses to high-dose (75 micrograms/kg) fentanyl anesthesia, we compared the circulatory effects of pancuronium (Pc) with those of vecuronium (Vc) in patients about to undergo coronary artery bypass surgery. The first measurements were made while the patients were awake. Thereafter the patients were anesthetized with fentanyl, intubated under succinylcholine relaxation and mechanically ventilated with a mixture of oxygen in air (FIO2 0.50). Ten minutes after completion of the fentanyl injection, the second set of measurements was made. Immediately thereafter, 0.1 mg/kg of either Pc (n = 10) or Vc (n = 10) was given, and no relaxant was administered to 10 control patients. Heart rate (HR) and cardiac index (CI), which had decreased significantly after fentanyl induction, decreased further after Vc, the latter decreases being significantly greater than the decreases in HR and CI observed in control patients. After Pc, HR and CI increased to control awake levels; rate-pressure product also increased and stroke index decreased. Five of ten patients receiving Vc had a HR below 45 beats/min; HRs this low were not seen in patients given Pc. Neither Pc nor Vc affected systemic vascular resistance, but filling pressures of the heart decreased abruptly after both relaxants. We conclude that if maintenance of preanesthetic hemodynamic status is the objective of anesthetic management of patients having coronary artery bypass surgery, Pc helps to achieve this objective during high-dose fentanyl anesthesia. On the other hand, many patients with limited coronary vascular reserve may well benefit from the negative chronotropic effect of Vc.