The in-vivo behaviour of microfabricated GOD (glucose oxidase)/ H2O2 glucose sensor implanted subcutaneously in normal anesthetized rats has been studied. The sensor consists of a planar, three-electrode microcell, an enzyme membrane (glucose oxidase and bovine serum albumin cross-linked with glutaraldehyde) and an outer diffusion limiting polyurethane membrane. The sensor behaviour during hyperglycaemic (13.8 mM and 11.2 mM), euglycaemic (7.8 mM) and hypoglycaemic (3.5 mM) plateau levels was determined. The values of the in-vivo sensitivity (0.64 \xb1 0.05 nA/mM) and background current (1.25 \xb1 0.4 nA) were determined using a two- point calibration method and then used to calculate apparent subcutaneous glucose concentrations. The results show the presence of a good correlation between all the plasma glucose levels (G) and the apparent subcutaneous tissue concentrations (G'), with G' = 0.997 . G - 0.066. r = 0.9782.