Based on previous clinical experience, an anesthetic technique for the rat, using ketamine, has been evaluated. The method comprised an i.v. bolus injection of 30 mg/kg for induction and an i.v. continuous infusion of 1.5 mg/kg/min for maintenance of anesthesia. Minor differences in ketamine metabolism between man and the rat are discussed. It appears that a higher ketamine concentration at the receptor site was required in the rat as compared to man. During ketamine anesthesia in the rat, fractional distribution of cardiac output and regional tissue perfusion were determined with the aid of the microsphere method. The study showed increased fractions to the heart, brain and tongue. The carcass fraction was elevated shortly after induction but reduced during steady-state anesthesia. Stomach, bowel and kidneys received reduced fractions early, but after these fractions returned to their initial levels. As a consequence of the increased cardiac output, regional tissue perfusion was increased in practically all organs.