We have determined that the mixed agonist-antagonist narcotic, butorphanol, improves CO2 response and ventilation after fentanyl anesthesia. A tentative dosage range has been established. Twenty-two patients were anesthetized with isoflurane, nitrous oxide, and fentanyl, which was continuously infused throughout the study. Postoperatively three 1-mg doses of butorphanol were administered IV. Blood pressure, heart rate, plasma epinephrine and norepinephrine concentrations, and pain intensity were essentially unchanged after butorphanol. Most of the improvement in breathing occurred after the first 1-mg dose. Mean respiratory rate increased from 7.8 +/- 5.0 to 11.0 +/- 4.8 min-1 (P less than or equal to 0.005), tidal volume increased from 469 +/- 302 to 844 +/- 390 ml (P less than or equal to 0.005), minute ventilation increased from 4.32 +/- 2.97 to 8.51 +/- 3.14 L/min (P less than or equal to 0.005), and the slope of the ventilatory response to CO2 increased from 0.36 +/- 0.37 to 0.90 +/- 0.80 L X min-1 X mm Hg-1 (P less than or equal to 0.05). Resting PaCO2 decreased from a baseline of 57.8 +/- 11.1 to 51.7 +/- 5.12 mm Hg (P less than or equal to 0.05) after the third dose.