Sixteen adult-onset diabetics, who were thought by current criteria to be well controlled on diet alone, had substantially elevated overnight, basal plasma glucose concentrations. Chlorpropamide had a sufficiently prolonged stimulatory effect on the beta cells such that normal basal plasma glucose levels were obtained in 13 patients. Both basal plasma C-peptide levels and the C-peptide response to meals became normal, with improved postprandial glycemia. Diurnal plasma triglyceride levels were reduced. Plasma growth hormone levels were normal both before and after therapy. When diet alone is insufficient to maintain basal normoglycemia in mild diabetes, chlorporpamide is as effective as basal insulin supplements in producing normal basal plasma glucose levels.