BACKGROUND: This study was designed to investigate the different analgesic potency and the action mechanism of the intrathecal isomers of ketamine. For these purpose, we evaluated the effect of intrathecal ketamine isomers on the behavioral response and the spinal c-fos expression in the formalin tested rats. METHOD: The subjects were divided into 2 groups(NF group, Fgroup). The NF group was designed for the purpose of the drug itself's effect on the induction of c-fos. Saline(NF/saline group), S(+) ketamine(NF/SK group), R(-) ketamine(NF/RK group) and ketamine(NF/K group) were administered intrathecally to be examined by immunocytochemical method. Same drugs were administered in the F group(F/saline, F/SK, F/RK, F/K) and formalin was injected into right hind paw of the rats after 30 minutes of intrathecal drug administration. The number of flinching was counted at intervals of 5 minutes for 60 minutes. In NF and F group, Fos immunoreactive neurons was counted after 2 hours of formalin injection and intrathecal drug injection respectively. RESULTS: In F/saline group, flinching was developed immediately after formalin injection and revealed biphasic response. The number of flinching in F/SK group, F/RK group, F/K group was significantly smaller than that of F/saline group. The number of flinching of F/SK group by comparison of F/SK vs F/RK was significantly smaller, and that of F/K group by comparison of F/RK vs F/K was significantly smaller. There was no significant difference among NF group on the total number of Fos immunoreactive neurons. In F group, Fos immunoreactive neurons increased significantly in comparison with NF group. The total number of Fos immunoreactive neurons in F/SK group, F/RK group and F/K group were significantly smaller than that of F/saline group. Of F group, the number of Fos immunoreactive neurons of F/SK was the smallest and F/K, F/RK followed increasing order. Attenuation of Fos induction by the ketamine isomers was observed in the whole spinal cord of F/SK group and F/K group but in the superficial and deep laminae of F/RK group. CONCLUSION: This study suggests that intrathecal ketamine isomers have an inhibitory effect on pathologic pain and c-fos expression in the rats and different analgesic effect which is lamina specific.