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The aim of this study was the development of a veterinary dosage form constituted by injectable biodegradable microspheres designed for the subcutaneous release of carboplatin, a chemotherapeutic drug. Poly(D,L-lactide) (PDLLA) microspheres were prepared by an emulsification/spray-drying method, using the drug-to-polymer weight ratios 19 and 15; blank microspheres (1% w/v) were prepared as a comparison. Microparticles were characterized in terms of morphology, encapsulation efficiency, and in vitro drug release behavior. In vivo tests were conducted on rats by subcutaneous injection of microsphere aqueous suspensions. Levels of carboplatin were evaluated both in the skin and in serum. The microparticles obtained had a spherical shape; particle size ranged from 5 to 7 μm, dependent on drug loading. Microspheres were able to control the in vitro release of the drug: approximately 90% to 100% of the carboplatin was released over 30 days. In vivo results showed that the microspheres were able to release high drug amounts locally, and sustained serum levels of drug were also achieved. Based on these results, carboplatin-loaded PDLLA microspheres may be useful for local delivery of the antineoplastic drug to the tumor, avoiding tumor recurrence in small animals, and may decrease the formation of distant metastases.