The effects of melatonin on physiological function remain unclear, although the therapeutic potential of melatonin is being increasingly recognized. The aim of the present study is to investigate the effects of exogenous melatonin on the spontaneous release of pituitary hormone in humans. A double blind placebo-controlled protocol was designed to examine 12 adult healthy volunteers and 12 sleep disorder patients who have been treating with low doses of melatonin for 1 year. Either exogenous melatonin or placebo of 1 mg was given at 09:00 hours, followed by the collection of blood samples every 20 min for 4 h. Each blood sample was examined for levels of serum melatonin, PRL, LH, FSH, GH and TSH. LH levels were higher in sleep disorder patients compared with the healthy volunteers. In other pituitary hormones, there were no significant difference between healthy adults and sleep disorder patients. In all subjects, PRL levels were stimulated by acute administration of 1 mg of exogenous melatonin, while the levels of other pituitary hormones were not affected. These results suggested that exogenous melatonin can affect the spontaneous release of LH and PRL in humans. In addition, we demonstrated that 1-year oral melatonin treatment did not affect the responses to the acute administration of melatonin.