['The baroreflex-mediated increase in heart rate (HR) in response to acute reduction of systolic blood pressure (SBP) was studied in order to assess whether the changes in arterial baroreflex sensitivity depend on the intensity of surgical stress, and location of visceral and somatic stimulation during surgery.', 'Patients were divided into visceral stimulation groups [upper (n = 30) and lower (n = 30) abdominal surgery] and somatic stimulation groups [upper (n = 25) and lower (n = 25) limbs, and chest wall (n = 25) surgery]. Acute hypotension as a baroreflex depressor test was induced by prostaglandin E1 (PGE1) 10 min before surgical incision (control) and during surgical manipulation under isoflurane-N2O anaesthesia or isoflurane-N2O-fentanyl anaesthesia. Plasma level of ACTH was measured in an additional 40 patients who underwent upper abdominal surgery.', 'During upper abdominal surgery, the heart rate baroreflex sensitivity (delta HR/delta SBP) was depressed from -0.47 +/- 0.05 (control) to -0.01 +/- 0.04 (P < 0.05). The reflex heart rate baroreflex sensitivity remained unchanged and was similar among the remaining groups of patients. The concentration of ACTH increased from 12.5 +/- 1.0 (control) to 343 +/- 78.6 pg.ml-1 (P < 0.05) with isoflurane-N2O anaesthesia but did not change with isoflurane-N2O-fentanyl anaesthesia during upper abdominal surgery.'] Does surgical stress attenuate reflex heart rate response to hypotension?