The short-term effect of the glucose-controlled insulin infusion system (GCIIS) Biostator on metabolic and hormonal responses was studied in 10 non-obese subjects with glucose intolerance and insulin low response to glucose. Glucose tolerance characterized by means of a 2 h glucose infusion test (12 mg/kg/min) primed by i.v. injection of 0.33 g glucose/kg body weight was completely normalized by GCIIS. Results provide further support that normalization of glucose tolerance by means of GCIIS is accompanied by peripheral hyperinsulinaemia if compared with 33 non-obese healthy controls. Glucose-induced endogenous insulin secretion (C-peptide) was significantly reduced during the GCIIS study possibly due to inhibition of insulin secretion by exogenous insulin and/or by lower blood glucose concentration after normalization of glucose tolerance. Acute normalization of glucose tolerance in these patients failed to alter pancreatic glucagon, NEFA and glycerol responses but normalized paradoxical growth hormone response to glucose.