Cisplatin incorporated into polylactic acid/polyethylene glycol acid blend polymeric microspheres was prepared as a dosage (CDDP-MS) by the solvent evaporation method in an oil-in-oil emulsion system. When CDDP-MS was preserved in phosphate-buffer saline, the dissolution rate of cisplatin from CDDP-MS was 14% after one day, 25% after 5 days, 33% after 7 days, 66% after 21 days and 85% after 30 days. CDDP-MS and CDDP aqueous solution (CDDP-SOL) were intraperitoneally administered to compare the tissue distribution of cisplatin in 42 rats each. On days 0.5, 1, 5, 7, 14 and 21, omentum, lung, liver and kidney were removed, and the CDDP concentration was measured. The CDDP concentration of the CDDP-MS group was maintained at a high level in the omentum for a long time. On the other hand, the CDDP level of CDDP-MS group was low in the lung, liver and kidney, compared with the CDDP-SOL group. Consequently, it was suggested that CDDP-MS is useful as a carrier in a drug delivery system, since it improves the burst effect and releases CDDP for a long time without serious side effects.