From June 1998 to August 1999, 39 California sea lions (Zalophus californianus) were immobilized at a rehabilitation center in northern California (USA) using medetomidine plus zolazepam and tiletamine (MZT), alone and in combination with isoflurane, with atipamezole reversal. Animals were given 70 μg/kg medetomidine with 1 mg/kg of a 1:1 solution of tiletamine and zolazepam intramuscularly. Mean (±SD) time to maximal effect was 5 ± 3 min. At the end of the procedure, animals were given 200 μg/kg atipamezole intramuscularly. Immobilization and recovery times were, respectively, 28 ± 18 and 9 ± 7 min for 15 animals maintained with MZT alone and 56 ± 47 and 9 ± 6 min for 18 animals intubated and maintained with isoflurane. One mortality occurred during anesthesia. Other disadvantages of the MZT combination included some prolonged ataxia, weakness and disorientation during recovery. However, the use of MZT resulted in faster induction and a more reliable plane of anesthesia that was reversible with atipamezole and safer than other previously used intramuscular agents. Physiological parameters including heart rate, respiratory rate, temperature, pulse oximeter saturation, and end-tidal carbon dioxide were monitored.