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The in-vivo behaviour of microfabricated GOD (glucose oxidase)/H₂O₂ glucose sensor implanted subcutaneously in normal anaesthetized 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 ± 0·05 nA/mM) and background current (1·25 ± 0·4 nA) were determined using a two-point calibrations. 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.