We studied insulin and C-peptide levels in patients with non-insulin-dependent diabetes mellitus (NIDDM) during standard oral or intravenous glucose tolerance tests (GTT) at the time of diagnosis and after 3 months dietary therapy. On the second occasion they also had an 'augmented' GTT, in which slow intravenous infusion of glucose raised basal plasma glucose to a level similar to that at the time of diagnosis. Eight patients had oral tests, and seven patients intravenous tests. In both groups, dietary therapy significantly reduced fasting and peak plasma glucose (p < 0.05 for oral; p < 0.01 for intravenous GTT). Serum insulin levels during conventional oral GTT were not significantly different after dietary therapy compared to diagnosis, but were significantly higher during the 'augmented' oral GTT (p < 0.05). In those patients who underwent intravenous GTT, there was a significant increase in both the total amount of insulin secreted (0-60 min) and in first-phase insulin secretion (0-10 min) during the 'augmented' test compared to diagnosis (p < 0.01), but first-phase insulin secretion during the conventional intravenous GTT was unchanged. Serum C-peptide responses were also greater during 'augmented' tests (p < 0.05), similar in pattern to serum insulin. There is a relative deficiency in insulin secretion in untreated NIDDM, which can be reversed by dietary therapy. It is essential to study insulin and C-peptide secretion in controlled 'fasting' glucose conditions.