Fentanyl was used to effectively produce neuroleptanalgesia in sea otters (Enhydra lutris) under field conditions when given intramuscularly at dosages of .05 to .11 mg/kg of body weight and in combination with azaperone at dosages of .11 to .45 mg/kg. Fentanyl at a dosage of .05 mg/kg, when combined with azaperone at a dosage of .20 mg/kg met most of the requirements for a safe, short acting, intramuscular immobilizing agent. The effects were readily reversible, there were no serious side effects and a wide safety margin.