The time course of the effects of an acute elevation in morning plasma cortisol on the daytime profiles of plasma glucose, serum insulin, and insulin secretion under constant glucose infusion was determined using a placebo-controlled design in two groups of eight normal men. In one group, the elevation of plasma cortisol was obtained by oral administration of 100 mg hydrocortisone. In the other group, the elevation was obtained by intravenous administration of 25 micrograms of corticotropin-releasing hormone. In both studies, the immediate effect of the increase in plasma cortisol, even when of very small amplitude, was an abrupt inhibition of insulin secretion without change in glucose concentration. Larger cortisol elevations, such as occurred after hydrocortisone administration, were additionally associated with the appearance of insulin resistance, which developed 4-6 h after the cortisol elevation and persisted for > 16 h. These observations support the concept that the 24-h cortisol rhythmicity is responsible, at least in part, for the normal diurnal variation in glucose tolerance.