Serum corticosteroid-binding globulin (CBG, transcortin) binding capacity for prednisolone and for cortisol, and levels of the circulating cortisol were evaluated in healthy adult subjects and in patients under long-term treatment with prednisone. Blood samples were drawn at 4-hr intervals throughout at least 24 hr; the prednisone administration was discontinued 24 hr before starting examination. Circadian fluctuations in the CBG-binding capacity for prednisolone occurred in healthy subjects: maximum binding was at 12 midnight and minimum binding at 8 a.m. The diurnal pattern of prednisolone-binding capacity was characterized by phase shifts of approximately 12 hr and 8 hr compared with those of serum cortisol and cortisol-binding capacity, respectively. There were no diurnal variations of prednisolone- and cortisol-binding capacity in the prednisone-treated patients. Moreover, the levels of both capacities, expressed as micrograms bound steroid/100 ml, were significantly reduced with respect to normal control subjects. Cyclic variations in serum CBG-binding capacity may represent an additional factor accounting for temporal differences in the action of synthetic corticoids which are bound by the transport protein. Synthetic derivatives when chronically administered may interfere not only with the adrenal secretion but also with the serum transport of glucocorticoids.