Forty-five patients underwent enflurane anaesthesia and surgery. Anaesthesia alone evoked little change in the plasma concentrations of ACTH, cortisol or antidiuretic hormone (ADH), but there were significant increases during surgery. The plasma concentrations of aldosterone increased during anaesthesia alone, and a further increase was noted during surgery. Neither enflurane anaesthesia nor surgery significantly influenced plasma concentrations of renin activity and thyroxine. A significant decrease in the plasma triiodothyronine concentrations was detected during anaesthesia alone, and a further decrease was found during and following surgery. Enflurane anaesthesia did not affect the plasma concentration of luteinizing hormone (LH) in male subjects throughout surgery but a significant decrease in female patients was detected to the first day after operation. The plasma concentrations of testosterone decreased during anaesthesia alone and surgery, and a further decrease was noted on the first day after operation. © 1979 Macmillan Journals Ltd.