Implantable, sustained release drug delivery devices offer benefits not obtained through oral ingestion or injection. These include delivery at a constant therapeutic rate, thus avoiding adverse intermittent and massive dose effects, as well as reliance upon patients taking their prescribed dosages. The drawbacks to their widespread acceptance have been their inability to maintain a zero-order release rate over an extended period of time and poor biocompatibility. Devices capable of satisfying these requirements have been developed and tested extensively for in vitro release of the narcotic antagonist cyclazocine. By using implant models prepared from Hydron, a hydrophilic polymer known to exhibit excellent tissue compatibility, we have found that the release rate could be precisely regulated by proper geometry, copolymer composition, concentration of ionogenic groups and cross-link density. Devices in such varied forms as capusles, barrier-film coated tablets and bulk polymerized rods have been tested in vitro for periods approaching 1 year.