Among the claimed advantages of moderate to high dose fentanyl as the principal anesthetic agent for coronary artery bypass graft (CABG) surgery is its general lack of postoperative respiratory depression. We reviewed our experience with this technique compared to a standard halothane technique in regard to requirement for postoperative mechanical ventilation. 19 of 24 patients in the halothane group and 8 of 24 patients in the fentanyl group were extubated after T-piece trial and the required time for T-piece trial were not statistically different (3.4±0.7 hours in halothane group and 4.0±0.5 hours in fentanyl group, mean ±SE). While the "successful" fentanyl patients whose mean dose of fentanyl was 39.9±10.5ug/kg (small dose, mean±S.E.) were extubated at about the same time postoperatively as the halothane patients (meank S.E.: 6.6±0.6 hours forfentanyl group and 5.7±0.5 hours for halothane group), the "unsuccessful fentanyl patients whose mean dose of fentanyl was 77.4±7,1ug/kg (large dose, mean±S.E.) were not extubated until 21.0±1.11 hours. We concluded that moderate to high dose fentanyl anesthesia supplemented with other IV agents does prolong the need for postoperative mechanical ventilation.