Ten common marmosets (Callithrix jacchus) and 10 black-tufted marmosets (Callithrix penicillata) were immobilized to compare the anesthetic effects of racemic ketamine and (S ) ketamine in combination with midazolam. The animals were distributed into four groups: five common marmosets (group CJR) and five black-tufted marmosets (group CPR) received 9.8 ± 1.4 mg/kg of racemic ketamine, and five common marmosets (group CJS) and five black-tufted marmosets (group CPS) received 10.4 ± 1.6 mg/kg of (S ) ketamine. All groups received similar dosages of midazolam (1.0 ± 0.15 mg/kg). During immobilization, heart rates, respiratory rates, rectal temperatures, and muscle relaxation scores were recorded at 5, 10, and 20 min after initial injection. Quality of induction and quality of recovery were evaluated in each marmoset by recording physical reactions including withdrawal reflexes, involuntary movements, salivation, compulsive licking, catalepsy, and ataxia. There were no significant differences in the induction, immobilization, and recovery times between the four groups. Similarly, there were no significant differences between groups in heart rates, respiratory rates, or body temperatures, although there was a significant decrease in respiratory rates over time in group CPR. In addition, the CJR and CPS groups showed significant decreases over time in rectal temperature. Muscle relaxation was more profound in the CPR group than in the other groups. Compulsive licking, involuntary movements, salivation, and withdrawal reflexes were observed more frequently in animals given S( ) ketamine; but in general, racemic ketamine and S( ) ketamine had similar effects in all callitrichines. Further studies are required to confirm that S( ) ketamine has different potency in these species.