In order to observe whether the improvement of diabetic control would promote insulin release of elder non-insulin-dependent diabetics, we performed oral glucose tolerance test and glucagon infusion in eight patients before and after fasting plasma glucose normalized with insulin therapy for three weeks. The levels of plasma glucose and C-peptide both on fasting and following intravenous glucagon infusion and oral glucose loading were measured. Their ages ranged from 60 to 72 years old. The initial fasting plasma glucose levels were 261 +/- 18 mg/dl (Mean +/- SEM). After insulin therapy for three weeks, the fasting plasma glucose levels dropped to 130 +/- 8 mg/dl (Mean +/- SEM, P less than 0.001). The fasting C-peptide/glucose ratio showed no difference before and after therapy. On the other hand, the C-peptidogenic index was markedly improved following insulin therapy, as compared with the value before therapy (P less than 0.05). These results suggested that greater C-peptide response was found in non-insulin dependent elderly diabetics following oral glucose after plasma glucose normalized with insulin therapy.