An implant containing a sustained release particle of fluorouracil, an antimetabolite fluoropyrimidine analog of the nucleoside pyrimidine, with antineoplastic activity. Upon implantation and subsequent release, fluorouracil is converted into the active metabolite 5-fluoroxyuridine monophosphate that competes with the pyrimidine uracil during RNA synthesis while another active metabolite, 5-5-fluoro-2'-deoxyuridine-5'-O-monophosphate, inhibits thymidylate synthase and thus DNA synthesis.