The use of medetomidine and ketamine, alone and in combination with isoflurane, with atipamezole reversal was evaluated for immobilizing 51 California sea lions (Zalophus californianus) for a variety of medical procedures at a rehabilitation center in northern California (USA) between May 1997 and August 1998. Animals were given 140 μg/kg medetomidine with 2.5 mg/kg ketamine intramuscularly. Mean (±SD) time to maximal effect was 8 ± 5 min. At the end of the procedure, animals were given 200 μg/kg atipamezole intramuscularly. Immobilization and recovery times were, respectively, 25 ± 12 and 9 ± 7 min for 35 animals maintained with medetomidine and ketamine alone and 58 ± 30 and 9 ± 9 min for 16 animals intubated and maintained with isoflurane. No mortalities occurred as a result of the immobilizations. Disadvantages of the medetomidine and ketamine combination included a moderate variation in time to maximal effect and plane of sedation, a large injection volume and high cost. However, this combination offers safe and reversible immobilization that can be easily administered by the intramuscular route and that produces a plane of anesthesia that is sufficient to carry out most routine diagnostic procedures.