To determine the mechanism by which ketone bodies decrease plasma glucose in man, seven normal postabsorptive volunteers were infused for 3 h with beta-hydroxybutyrate. Total plasma ketone bodies (beta-hydroxybutyrate plus acetoacetate) increased to levels (approximately 2.5 mM) observed after a 2- to 3-day fast in normal subjects. Plasma glucose decreased 10% concomitant with decreases of 25% and 10%, respectively, in the rates of glucose production and glucose utilization determined isotopically with [3-3H]glucose. Plasma insulin and glucagon concentrations were unaltered, but plasma C-peptide levels increased from 2.6 +/- 0.1 ng/ml to a maximum of 3.9 +/- 0.2 ng/ml at 30 min (P < 0.01) and remained significantly increased for more than 2 h. Plasma alanine decreased approximately 14% (P < 0.05), while plasma lactate increased 25% (P < 0.01) so that there was no net decrease in the combined levels of these gluconeogenic substrates. These results demonstrate that physiological increments in circulating ketone body concentrations decrease plasma glucose in normal man by suppressing glucose production, an effect which can be explained by the stimulation of insulin secretion being reflected only in changes in plasma C-peptide. Thus, changes in pancreatic B cell function not sufficient to alter peripheral plasma insulin levels may cause significant changes in hepatic glucose production.