Plasma cortisol, beta-endorphin immunoreactivity (PBEir) and arginine vasopressin (AVP) responses during and after the continuous infusion of fentanyl or alfentanil were studied in 19 patients undergoing coronary artery bypass grafting (CABG). Plasma cortisol concentration decreased significantly in both groups during the anaesthesia and surgery before cardiopulmonary bypass (CPB); an increase was evident during CPB in both groups, but a statistically significant increase was not observed during the rest of the study, including the awakening from anaesthesia. PBEir increased with both opiates immediately after initiation of CPB and remained so during the rest of the study. There were no significant changes in plasma AVP concentrations during anaesthesia and surgery. After discontinuation of opiate infusions, an increase in AVP concentration commenced earlier in the alfentanil group than in the fentanyl group. At awakening from anaesthesia, a significant correlation was observed between log plasma AVP concentration and systemic vascular resistance. It is concluded that, with continuous fentanyl and alfentanil infusions in a total dose relationship of 1:13 in patients undergoing CABG, cortisol and AVP responses to surgery and CPB can be suppressed. However, during recovery from anaesthesia, the attenuating effect of alfentanil seems to wear off more rapidly than that of fentanyl. PBEir response to CPB and emergence from anaesthesia could not be prevented with either analgesic.