This study was performed to investigate the effects of nitrous oxide-narcotic anesthesia (Nitrous oxide-pethidine HCI-muscle relaxant) and surgery on plasma aldosterons level in surgical patients and the aldosterone level in plasma was measured by the Abbott Laboratorie's Aldosterone RIA Kit. Average body weights and ages of subjects were 58.1±6.45(kg) and 43.8±11.55(yrs) respectively. Mean control aldosterone level in plasma was 121.3±58.66I(pg/ml) (mean ± SE) but it gradually rose during anesthesia and surgery. The mean plasma aldosterone level rose to 232.7±40.06(pg/ml) (p<0.005) at 90 minutes after the start of surgery, and therefore, it was a significantly higher value than that of the control.