The dynamic relationship of glucose concentrations and insulin secretion during the postabsorptive state is complex and has been associated with a variety of cyclic rhythms. To study the pattern of insulin and glucose response immediately after a mixed meal, we collected blood every 15 min from 0730 to 1645 h from eight normal resting men (age 24.9 ± 2 . 1 yr). They took identically constituted mixed meals at 0800 and 1145 h. Concentrations of glucose and insulin were measured in samples taken throughout the study, whereas levels of C-peptide, glucagon, and a-NH2 were determined in samples taken after 1130 h only. Computer-assisted analysis was used to identify significant increments and declines in concentrations and to quantify the coincidence of peaks of glucose, C-peptide, glucagon, and α-NH2 with peaks of insulin. Coefficients of correlation between data points were calculated for each individual. The patterns of blood insulin and glucose after breakfast and lunch were different. After breakfast, a single simultaneous peak in insulin and glucose occurred ∼60 min after starting the meal. In contrast, the pattern after lunch in seven of the eight subjects was clearly biphasic. There were secondary, significant coincident peaks in serum insulin, glucose, and C-peptide occurring 1.75—2.25 h after the meal was served. The secondary peak appeared unrelated to the late absorption of protein because it was not associated with consistent changes in serum a-NH2 concentration. Erratic variations characterized the postlunch pattern of glucagon levels, excluding a role for this counterregulatory hormone in the control of the biphasic insulin and glucose response. Of the measured substances, only glucose, insulin, and C-peptide demonstrated significant cross-correlations. These data indicate that in this well-defined group of healthy volunteers, the response of plasma glucose, serum insulin, and C-peptide to lunch is biphasic.