Serum immunoreactive insulin responses to meal stimulus were studied in 20 newly detected non insulin dependent diabetes mellitus patients, following one week of treatment with high carbohydrate, high fibre diet and glibenclamide. Ten patients showed "rapid glycaemic response" i.e. the glycaemic response was good within a week. The rest of them were called "slow responders". The insulin responses were heterogenous. Mathematical calculations using the glucose and insulin responses showed improved beta cell function and peripheral action of insulin in rapid responders. On the other hand, the slow responders showed only slightly improved beta cell function with no change in peripheral action of insulin. The second phase of the study constituted follow-up studies upto 6 months. The corrected insulin response (CIR) increased initially in several patients. The peripheral insulin action improved in all patients with longer duration of treatment and lower insulin concentrations were required to maintain normoglycaemia at this stage. The results of the study indicate that a) multiple factors influence glucoregulation, b) even short term effects of the drug appear to be mediated by extra pancreatic mechanisms, and c) the extrapancreatic action improves significantly on long term use of the drug.