Polycarbonate microspheres containing dibucaine were prepared and evaluated as injectable sustained release formulations. Poly (ethylene carbonate) and two lots of poly (ethylene propylene carbonate) were used alone or as mixtures to prepare microspheres. The dibucaine contents of these microspheres were about 27 to 30% when they were prepared at a constant drug/polymer ratio of 30/70. The release rate of dibucaine from the microspheres could be varied by changing the composition of polymer matrices. Therefore, various sustained release patterns of dibucaine were obtainable by the employment of polycarbonate microspheres of different polymer compositions. Three representative preparations of microspheres were examined for local anesthetic effects in guinea pigs. The longest duration of local anesthetic effect was obtained following the implantation of the poly (ethylene propylene carbonate) microspheres with an intermediate release rate of the drug in vitro.