This study was undertaken to determine if fentanyl and sufentanil could produce dose-related suppression of hemodynamic and hormonal responses to surgical stimulation. Eighty patients scheduled for elective CABG were studied in two consecutive protocols: protocol I was a randomized double-blind study of 40 patients who received a single dose of fentanyl (50 or 100 μg/kg) or sufentanil (10, 20, or 30 μg/kg). Hemodynamic measurements and hormonal concentrations (renin, aldosterone, cortisol, and catecholamines) were determined before and after induction and after intubation and sternotomy. Protocol II was an open randomized study of 40 patients who received sufentanil in one of four doses: 30 μg/kg as a single dose, 10 μg/kg plus infusion 0.05 μg·kg-1·min-1, 20 μg/kg plus infusion 0.1 μg·kg-1· min-1, or 40 μg/kg plus infusion 0.2 μg·kg-1· min-1. Hemodynamic measurements and plasma sufentanil and catecholamine concentrations were determined before and after induction and after intubation, sternotomy, and aortic cannulation. Both protocols defined a hemodynamic response as a 15% or more increase in systolic blood pressure (SBP) from control and a hormonal response 50% or more increase over control. During protocol I, 18 patients had a hemodynamic response (average increase in SBP 22.6±2%) and 35 patients had a total of 59 hormonal responses. During protocol II, 24 patients had a hemodynamic response (average increase in SBP - 31±3%) and there were 15 catecholamine responses. There were no differences between dose groups in either protocol. It was concluded that in these dose ranges, suppression of hemodynamic or hormonal stress responses is not related to opioid dose. Furthermore, the maintenance of high plasma opioid concentrations by opioid infusions does not decrease the incidence of these responses.