Nine non-obese males with non-insulin-dependent diabetes mellitus (NIDDM) were evaluated before and after 3 and 12 months (6 patients) treatment with the second generation hypoglycemic sulfonylurea: gliclazide. They underwent an oral glucose tolerance test, intravenous glucose and arginine tests measuring plasma insulin and C-peptide responses. Pre-hepatic insulin production and insulin delivery to peripheral tissues were calculated by deconvolution techniques and hepatic extraction of insulin estimated. An improvement was observed in the beta-cell function of the patients on gliclazide treatment: reduction of fasting plasma glucose associated with a progressive increase in C-peptide level but insulin levels decreased at 12 months, suggesting an increase in hepatic insulin extraction at this time. In the same way, while plasma glucose values after oral and i.v. glucose were greatly reduced at 3 and 12 months treatment, insulin did not change but C-peptide levels increased significantly at 12 month treatment. While the prehepatic insulin secretion rate increased progressively on gliclazide during all glucose challenges, the fractional hepatic insulin extraction fell after 3 and increased at 12 month treatment, with opposite changes in insulin delivered to peripheral tissues. Thus the insulinogenic effect of gliclazide could be masked during long-term administration by a concomitant effect of gliclazide which increases hepatic extraction of insulin. The maintenance of the responsiveness to the non-glucose secretagogue, arginine, as evaluated by the C-peptide levels, before and after correction of hyperglycemia, suggested improvement of beta-cell sensitivity to glucose after sulfonylurea treatment.(ABSTRACT TRUNCATED AT 250 WORDS)