The rhythmicities observed in the plasmatic levels of cortisol are generally attributed to rhythms of production and release of the hormone. Since the plasmatic concentration of any given substance is a function of its production and its removal from the circulation, it is conceivable that the metabolism of cortisol also occurs in an oscillating fashion. To test this hypothesis Rhesus monkeys were submitted to bilateral adrenalectomy; cortisol was replaced at a constant infusion rate while blood was sampled at hourly intervals for the measurement of cortisol plasma levels. Rhythmic oscillations in the cortisol levels were observed. These rhythms exhibited two major components: a circadian and an ultradian component. The authors suggest that these rhythms be considered whenever normal or pathologic hormone rhythmicities are analyzed.