The objective of this study was to evaluate whether first-degree relatives (FDRs) of patients with type 2 diabetes had abnormal circadian insulin secretion and, if so, whether this abnormality affected their glucose metabolism. Six African-American FDRs with normal glucose tolerance and 12 matched normal control subjects (who had no family history of diabetes) were exposed to 48 h of hyperglycemic clamping (approximately 12 mmol/l). Insulin secretion rates (ISRs) were determined by deconvolution of plasma C-peptide levels using individual C-peptide kinetic parameters. Detrending and smoothing of data (z-scores) and computation of autocorrelation functions were used to identify ISR cycles. During the initial hours after start of glucose infusions, ISRs were approximately 60% higher in FDRs than in control subjects (585 vs. 366 nmol/16 h, P < 0.05), while rates of glucose uptake were the same (5.6 mmol x kg(-1) x h(-1)), indicating that the FDRs were insulin resistant. Control subjects had well-defined circadian (24 h) cycles of ISR and plasma insulin that rose in the early morning, peaked in the afternoon, and declined during the night. In contrast, FDRs had several shorter ISR cycles of smaller amplitude that lacked true periodicity. This suggested that the lack of a normal circadian ISR increase had made it impossible for the FDRs to maintain their compensatory insulin hypersecretion beyond 18 h of hyperglycemia. As a result, ISR decreased to the level found in control subjects, and glucose uptake fell below the level of control subjects (61 vs. 117 micromol x kg(-1) x min(-1), P < 0.05). In summary, we found that FDRs with normal glucose tolerance had defects in insulin action and secretion. The newly recognized insulin secretory defect consisted of disruption of the normal circadian ISR cycle, which resulted in reduced insulin secretion (and glucose uptake) during the ascending part of the 24 h ISR cycle.