Diurnal Rhythm in Plasma Corticosterone and Lack of Diurnal Rhythm in Plasma Compound F-like Material in the Rat.∗

Summary Concentration of corticosterone in blood of the rat varied as a function of time of day. Highest levels of corticosterone were found between 2 and 3 p.m., and 9 and 10 p.m. There was no diurnal variation in concentration of an unknown component of adrenal origin found in blood. These data supply additional evidence for presence of a diurnal rhythm in secretory activity of the adrenal cortex and support the concept that corticosterone is the glucocorticoid of physiological significance in the rat. Samples of corticosterone and 17-hydroxycorti-costerone were supplied through courtesy of Merck, Rahway, N. J., and Upjohn Co., Kalamazoo, Michigan.