The MAC($ED_{50}$)values of enflurane, fentanyl, and nalbuphine-enflurane under 65% $N_2O$ were determined in 76 Sprague-Dawley rats using the tail-clamp technique to compare the equipotent effects of intravenous and inhaled anesthetics. The rats were divided into 3 groups: enfluarne, fentanyl, and nalbuphine-enflurane. Results were as follows: 1) The MAC value of enflurane under 65% $N_2O$ was $1.160{pm}0.05%$ and after subcutaneous nalbuphine 20 mg/kg injection, the values were 1.08% at 60min and 0.99% at 90min. 2) The lowest $ED_{50}$ for fentanyl was 26.8 ${mu}g$/kg at 15 min, and the $ED_{50}$, 30, 45, and 60min after the injection were 36.2, 39.7, and 44.7 ${mu}g$/kg, respectively. 3) On arterial blood gas analysis under 65% $N_2O$-1MAC($ED_{50}$), fentanyl and nalbuphine-enflurane groups showed mild increase in $PaCO_2$, but there were no significant differences among 3 groups. Fentanyl group showed significant difference in pH compared with enfluarane and nalbuphine-enflurane groups. 4) Rats injected with high dose fentanyl(above $40{mu}g$) displayed rigidity and respiratory depression.