Résumé : Peripheral plasma or serum concentrations of glucose, insulin, C-peptide, glucagon, and cortisol and insulin secretory rates (ISR) were determined at 15-min intervals in eight normal subjects during a constant iv infusion of 4.5 mg glucose/kg� min for a 24-h period. During each sampling interval, the secretory rate of insulin was calculated by deconvolution of the peripheral plasma C-peptide concentration using C-peptide kinetic parameters derived after bolus injections of C-peptide in individual subjects. Periodogram analysis of the individual glucose curves demonstrated a circadian rhythm in all subjects, with a major nocturnal acrophase occurring at an average clock time of 0228 h (range, 0045-0350 h). In five of the eight subjects, a minor acrophase occurred at an average time of 1774 h (range, 1530-2045 h). This diurnal variation in plasma glucose levels was not paralleled by a similar pattern in insulin secretion. Although glucose was infused at a constant rate, significant pulses were found in glucose, insulin, and C-peptide levels and ISR; the pulse durations of these parameters were 182 ± 30 (± SE), 89 ± 5, 100 ± 8, and 85 ± 5 min, respectively, and their periodicities were 208 ± 33, 106 ± 7, 114 ± 10, and 106 ± 7 min. The durations and frequencies for pulses of insulin, C-peptide, and ISR were not significantly different, whereas glucose pulses had a longer duration and were less frequent (P < 0.05, by analysis of variance). On the average, 54 ± 9% of the C-peptide pulses and 47 ± 8% of the ISR pulses were concomitant with a pulse in glucose levels. Moreover, approximately half of the Cpeptide and ISR pulses that were not concomitant with a glucose pulse occurred in synchrony with a shoulder on the up-stroke or down-stroke of glucose pulses. Analysis of glucagon and cortisol profiles revealed no significant associations with the insulin and glucose oscillations. In conclusion, during a constant glucose infusion in normal subjects, regular oscillations of insulin secretion occur at 80- to 120-min intervals. Their tight coupling with glucose oscillations and the lack of association with fluctuations of glucagon and cortisol suggest that these oscillations represent a dynamic property of the insulin-glucose feedback loop. (J Clin Endocrinol Metab 67: 307, 1988). © 1988 by The Endocrine Society.