A potentially confounding variable inherent in studies designed to examine the effect of melatonin administration in humans is the presence of an endogenous melatonin rhythm in the experimental subjects. The effects of exogenous melatonin administration on serum hormone rhythms was recently examined in a male patient who lacked detectable circulating levels of endogenous melatonin. The patient's pineal gland had been destroyed five years previously in the course of treatment for a pineal astrocytoma. On three separate occasions, over approximately a one-year period, the patient was given daily oral melatonin replacement (2 mg/day, 1 mg/day and 0.5 mg/day). These experiments were designed to assess the effects of exogenous melatonin on serum growth hormone, prolactin, cortisol and testosterone rhythms. Analysis of blood samples collected every 2-4 hours periods both before and during melatonin replacement revealed that the exogenous melatonin rhythm was associated with improvements in self-reported sleep and mood ratings. Melatonin administration produced robust nocturnal peaks in serum growth hormone and prolactin levels immediately following ingestion of the hormone, while serum cortisol and testosterone rhythms were not influenced. These results suggest that melatonin may modulate the coordination and enhancement of selected biological rhythms in man.