Seven captive adult male guanacos (Lama guanicoe) weighing 112.0 ± 10.9 kg (mean ± standard deviation) were anesthetized with a combination of medetomidine (90.0 ± 8.8 µg/kg), ketamine (2.7 ± 0.3 mg/kg), and butorphanol (0.3 ± 0.03 mg/kg) administered intramuscularly to evaluate its anesthetic and cardiopulmonary effects. Inductions were smooth and rapid, with a mean time to initial effect of 3 ± 1.5 min and a mean time to recumbency of 5.1 ± 3.1 min. Anesthesia was predictable, smooth, and characterized by excellent muscle relaxation. Spontaneous ventilation was maintained throughout anesthesia in all animals. Marked bradycardia ranging from 24 to 52 beats/min was noted across all time points for all individuals. Median heart rates decreased during the procedures, but median heart rate, temperature, respiratory rate, and end-tidal carbon dioxide values over the 20 min monitoring period were not significantly different. Mean arterial partial pressure of oxygen (PaO₂) on initial sampling was 65.9 ± 14.8 mm Hg, with six of seven animals exhibiting hypoxemia (PaO₂ < 80 mm Hg). After oxygen supplementation for 20 min, mean PaO₂ values showed statistically significant increases to a mean value of 127.7 ± 32.4 mm Hg (P  =  0.0014). Mean arterial partial pressure of carbon dioxide (PaCO₂) showed a significant increase over the monitoring period (P  =  0.0004), and mild hypoventilation (PaCO₂ > 45 mm Hg) was noted in four animals. Mean total duration of procedure time was 23.3 ± 3.3 min. All guanacos received 0.45 ± 0.04 mg/kg atipamezole and 2.7 ± 0.25 mg/kg naltrexone administered intramuscularly for anesthetic antagonism. Recoveries were smooth and uncomplicated. Mean time to sternal recumbency after antagonist administration was 7.7 ± 4.5 min, and time to successful standing was 12.9 ± 5.0 min, with all animals standing on first attempt.