(7-O-(2,6-Dideoxy-2-fluoro-alpha-L-talopyranosyl)adriamycinone-14- hemipimerate (ME2303) showed a more marked growth inhibition of Lewis lung carcinoma than adriamycin (ADM). When administered to s.c. Lewis lung carcinoma-bearing mice, ME2303 in the plasma and liver was rapidly metabolized and disappeared. However, ME2303 was incorporated into the tumor at higher concentrations and remained in the tumor for a longer period than in the plasma and liver. ME2303 was metabolized to 7-O-(2,6-dideoxy-2-fluoro-alpha-L-talopyranosyl)adriamycinone (M1), the product of esterolysis, and its reduced derivative at the C-13 position (M2). Larger amounts of these metabolites were found in the analyzed tissues than in plasma. The maximum concentration of M1 in the tumor was observed at 2 h posttreatment, while the maxima in the plasma and liver were observed at 15 min. On the other hand, i.v. injection of M1 into mice showed a weaker antitumor effect than ME2303 injection, though M1 levels in the plasma and tumor were almost the same as those after administration of ME2303 at the maximum tolerated doses. Some metabolites of ME2303 were found in the tumor after administration of ME2303, but not after administration of M1. ADM remained in the analyzed tissues for a long period and ADM concentrations in the tumor were much higher than in the plasma but less than in the liver. M1 reached a concentration higher than that of ADM in the tumor, opposite to the pattern observed in the liver. The conversion process from ME2303 to M1, the metabolites and their locations in the tumor may be important for the marked antitumor effect of ME2303 in vivo.