It's an idea, but I find the use of silastic implants too inaccurate (any control of the steroid diffusion quality, manual production = each implant is different). Personally, I prefer to treat my animals by Alzet implant (ei: for the rat: Alzet 2004, manufacturer's data: nominal duration 4 weeks, continius infusion of 0.25μl / hr). But a little expensive...