Thiafentanil oxalate, previously known as A-3080, is a synthetic opioid used for chemical immobilization of a variety of nondomestic hoofstock species.1–4,8,10,12 This study compared the combination of thiafentanil oxalate, medetomidine, and ketamine (TMK; 0.09 ± 0.02 mg/kg, 0.01 ± 0.003 mg/kg, and 1.36 ± 0.33 mg/kg, respectively) with the combination of medetomidine and ketamine (MK; 0.09 ± 0.02 mg/kg and 3.48 ± 0.55 mg/kg, respectively) for anesthetization of 17 captive male axis deer (Axis axis) for vasectomy. Nine deer received TMK and eight deer received MK via projectile syringe during the months of January and February, 2005. Mean induction and arousal times, vital signs, and arterial blood gas values were monitored and compared. All animals received supplemental oxygen during the surgical procedure. Animals receiving TMK were reversed with naltrexone (100 mg/mg thiafentanil) and atipamazole (5 mg/mg medetomidine). Animals receiving MK were reversed with atipamazole (5 mg/mg medetomidine). Two MK animals and three TMK animals required supplementation with ketamine i.v. immediately upon handling. Six of the nine animals immobilized with TMK required intubation for positive-pressure ventilation. Two of these six animals also required isoflurane to maintain anesthesia. Mean induction time was 3.5 ± 2.0 min in the TMK group, and 9.8 ± 6.7 min in the MK group. Despite shorter mean induction times, animals anesthetized with TMK experienced unpredictable inductions, apnea, muscle rigidity, limb movement, and significant respiratory and metabolic lactic acidosis. MK resulted in smoother inductions, better respiratory function, and less adverse metabolic disturbances, and thus was considered superior to TMK for anesthesia in captive axis deer at the dosages tested.