Insulin secretion and insulin resistance were examined in seven obese type II diabetics before and after control of plasma glucose levels without weight loss. Control was achieved by regular insulin injection (60-205 U/day in four doses). After 10 days of therapy, plasma insulin and C-peptide responses to oral glucose were significantly improved. Insulin-induced glucose rates, estimated by the glucose clamp technique, averaged 1.08 +/- 0.30 mg/kg. min (mean +/- SEM; n = 7) before treatment and were unchanged (1.08 +/- 0.25) after treatment. These indicate that short term control of plasma glucose improved insulin secretion but not insulin sensitivity. The impaired insulin secretion and insulin sensitivity in type II diabetics appears to be, in part, secondary to metabolic abnormalities associated with hyperglycemia.