Eight horses were anesthetized three times, by intravenous administration of xylazine (1.1 mg/kg) and ketamine (2.2 mg/kg), detomidine (0.02 mg/kg) and tiletamine-zolazepam (1.1 mg/kg), or detomidine (0.04 mg/kg) and tiletamine-zolazepam (1.4 mg/kg). The sequences were randomized. The duration of analgesia and the times to sternal and standing positions were recorded. Heart rate, arterial pressure, pHa, PaCO2, and PaO2 were measured before and during anesthesia. The duration of analgesia with the two doses of detomidine-tiletamine-zolazepam, 26 +/- 4 minutes and 39 +/- 11 minutes, respectively, was significantly longer than the 13 +/- 6 minutes obtained with xylazine-ketamine. Bradycardia occurred after administration of detomidine, but heart rates returned to baseline values 5 minutes after administration of tiletamine and zolazepam. Arterial pressure was significantly higher and PaO2 significantly lower during anesthesia with detomidine-tiletamine-zolazepam than with xylazine-ketamine. Some respiratory acidosis developed with all anesthetic combinations. The authors conclude that detomidine-tiletamine-zolazepam can provide comparable anesthesia of a longer duration than xylazine and ketamine, but hypoxemia will develop in some horses.