Plasma concentrations of alpha-atrial natriuretic peptide (alpha-ANP), antidiuretic hormone (ADH) and aldosterone (ALDS) were determined by radioimmunoassay in 9 patients undergoing aortocoronary bypass grafting under high dose fentanyl (94.4 micrograms.kg-1) anesthesia. These three levels in pre-anesthetic period (control values) were within normal ranges suggesting the absence of congestion and dehydration. Although these values changed significantly after sternotomy, they all increased at the termination of cardiopulmonary bypass (CPB) reaching 2.5 fold in alpha-ANP, 27.7 fold in ADH and 2.4 fold in ALDS as compared with control (P less than 0.05). Present results indicate that high dose fentanyl anesthesia cannot suppress ADH and ALDS level during CPB as was previously demonstrated and the observed rise in alpha-ANP level is considered to be inadequate not only for diuresis but also for vascular dilatation. Administration of alpha-ANP to ameliorate circulatory insufficiency after CPB should probably be considered.