The MAC(ED)values of enflurane, fentanyl, and nalbuphine-enflurane under 65% NO were determined in 76 Sprague-Dawley rats using the tail-clamp technique to compare the equipo-tent 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% NO was l.l60.05% and after subcutaneous nalbuphine 20 mg/kg injection, the values were I.08% at 60min and 0.99% at 90min. 2) The lowest ED for fentanyl was 26.8 μg/kg at 15 min, and the ED 30, 45, and 60min after the injection were 36.2, 39.7, and 44.7 μg/kg, respectively. 3) On arterial blood gas analysis under 65% NO-1MAC(ED), fentanyl and nalbuphine- enflurane groups showed mild increase in PaCO2, 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 μg) displayed rigidity and respiratory de- pression.