Six adult dogs were anesthetized by deep hypothermia with a combination high dose fentanyl and PGE1 (F group). The circulatory system and blood catecholamine concentration were determined. Another 6 adult dogs were anesthetized by deep hypothermia with a combination ether and PEG1 (E group) for a comparison with the F group. Before cooling under anesthesia, the PAP, PVRI and Pp/Ps showed a significant difference in the E group. During cooling, the MAP was 74% and 56% of the control values in the F and E groups, respectively and there were significant differences between the F and E groups. The peak dp/dt/IP was significantly different between the F and E groups at the lowest temperature (46% and 27% of the conrol values, respectively). The CI was 34% and 25% of control values at the lowest temperature in both groups. EP or NE did not significantly increase during cooling or rewarming in the F group. The EP had a tendency to increase during rewarming and NE significantly increased in comparison with the control values during cooling and warming in the E group. Combination anesthesia with high dose fentanyl and PGE1 inhibits the circulatory systems to a lesser degree, resulting in easier maintenance of the circulatory system. This combination system is also more stable than that with ether and PGE1 or with ether and TFP.