Biosensors are miniaturized analytical tools which comprise a biological detection element providing specificity to the analyte, and a physical transducer which guarantees an output signal, e.g. an electric current the size of which is proportional to the concentration of the analyte. They provide the unique possibility of continuous in vivo monitoring. Glucosensors were in fact the first biosensors under study. Among them, the most advanced devices are measuring amperometrically the hydrogen peroxide generated in a stoichiometric relation to the prevailing glucose concentration during glucose oxidase-mediated glucose oxidation. They proved useful in commercially available glucose analyzers and in experimental subcutaneous monitoring. Here it is shown (a) that under steady state conditions the s.c. glucose concentration is nearly identical to that in blood, (b) that s.c. inserted glucose electrodes do mirror the intracorporal glucose concentration both under hypo-, normo-, and hyperglycaemic conditions with a clinically relevant accuracy, (c) that even stable feedback control of intracorporal glucose concentration is possible employing s.c. glucosensor signal as an input to automated insulin pump controller, and (d) that stable function of s.c. sensor is usually accomplished over intervals up to one day but in some cases applications over up to ten days could be realized. The underlying problem consists in an insufficient functional biostability which is a function of biocompatibility and size of the sensor, of its sterility, and of the permanent skin penetration. The latter is still required to get the device in place, to keep it in function, and to make use of the data under any condition.(ABSTRACT TRUNCATED AT 250 WORDS)