Cardiovascular effects of high dose fentanyl-O2 during induction of anesthesia for coronary artery bypass surgery were studied in 10 A.S.A. class III patients. All patients had normal left ventricular function at cardiac catheterization and had ejection fractions greater than or equal to 0.60. Cardiovascular profiles, obtained after the stepwise intravenous administration of 15, 30, and 50 microgram/kg of fentanyl and compared with base line values, showed statistically significant decreases in cardiac and left ventricular stroke work indices. Plasma samples were assayed radioenzymatically for norepinephrine, epinephrine, and dopamine. Plasma norepinephrine levels were elevated significantly after 15 microgram/kg, remained elevated after 30 microgram/kg, but after 50 microgram/kg declined toward base line values. Mixed venous oxygen content decreased significantly after 50 microgram/kg of fentanyl. From these data, it is concluded that although high dose fentanyl-O2 is suitable for induction of anesthesia in patients with ischemic coronary artery disease, it may be associated with decreasing left ventricular function at doses in excess of 30 microgram/kg. The decrease in mixed venous oxygen content excludes decreasing metabolic demands as a sole reason for decreases in cardiac and left ventricular stroke work indices. The catecholamine response observed suggests either a dose- or time-related effect of fentanyl or the "stress" of induction of anesthesia.