Backgroud: Systemic administration of opioid can prolong the duration of epidural anesthesia. The authors examined the effect of nitrous oxide ($N_2O$) on the level of sensory block induced by epidural lidocaine. Methods: Twenty minutes after epidural injection of 2% lidocaine (below 70 years : 20 ml, 70 years and above : 15 ml), the level of sensory block was assessed (2nd stage). Patients were randomly assigned to receive either medical air (control group, n=15) or 50% $N_2O$ in oxygen ($N_2O$ group, n=15) for 10 minutes, the level of block was reassessed (3rd stage). Pateints were given room air (control group) or 100% oxygen for 5 minutes and room air for 5 minutes ($N_2O$ group), and the level of block was reassessed (4th stage). Results: At the 3rd stage, $N_2O$ group showed 4.3 cm cephalad increase in the level of sensory block (p=0.005), but control group revealed 1.43 cm regression. After discontinuation of gas, the level of block regressed in both group (p=0.000). At the 4th stage, $N_2O$ group revealed 3.5 cm cephalad increase (p=0.048) and control group 1.97 cm regression (p=0.001) as compared with the 2nd stage. Conclusions: The level of sensory block induced by epidural lidocaine was significantly increased cephalad by concommitant use of 50% $N_2O$ for 10 minutes.