The following three regimens of anesthesia in mice were compared: (1) Ketamine 100 mg--xylazine 5 mg/kg b.wt. i.m., (2) pentobarbitone 50 mg/kg b.wt. i.p., and (3) carfentanyl 0.003 mg--etomidate 15 mg/kg b.wt. i.m. For these dosage rates the respiratory variables, i.e., respiratory rate, paO2, paCO2, pHa, BEa, HCO-3a, and the circulatory parameters, i.e., heart rate, mean arterial pressure and hematocrit, were determined during the conscious state, during surgical anesthesia, and at waking time. With ketamine and xylazine, the respiration was moderately decreased whereas the cardiovascular system was strongly depressed. Pentobarbitone induced a high respiratory depression but a lesser degree of circulatory depression. Analgesia was inadequate. Although there was a moderate respiratory and circulatory depression during anesthesia with carfentanyl and etomidate the drug-induced excitation and muscle spasms do not recommend this anesthetic combination for mice. Of these three methods, the combination of ketamine and xylazine is considered the most reliable for anesthesia of mice.