OBJECTIVE In spite of broad interest, intensive studies on function of melatonin have not yielded much information about relationships between this hormone and kidneys in health, and particularity, in disease. There are only a few studies dealing with melatonin concentrations in renal diseases, mainly performed in hemodialyzed patients with end-stage renal disease (ESRD). Moreover, the most melatonin assays were performed during the daytime, and the results are conflicting. Therefore, the aim of the present study was to determine the circadian melatonin profiles in patients ESRD before and after hemodialysis. METHODS Thirty patients (19 males and 11 females) with ESRD undergoing dialysis, aged 22 to 64 years (mean+/-SEM: 49.1.0+/-1.9 years) were included in the study. The control group consisted of 20 healthy volunteers (13 males and 7 females) aged 35 to 55 years (mean+/-SEM: 46.2+/-1.4 years) matched according to sex and age. Blood samples were collected on the day preceding hemodialysis and one day following dialysis at 08:00, 12:00, 16:00, 20:00, 24:00, 02:00, 04:00, and 08:00 h. Melatonin concentration was measured by enzyme immunoassay. RESULTS In patients with renal insufficiency undergoing dialysis mean melatonin nocturnal concentrations were significantly lower then those in healthy volunteers. The presence of the circadian rhythm in melatonin concentrations (although of significantly lower nocturnal amplitude) was detected only in 8 patients with renal failure undergoing hemodialysis, whereas in remaining 22 patients no such rhythm was found. Hemodialysis did not influence melatonin concentrations. CONCLUSIONS The mechanism of depressed melatonin concentrations in hemodialyzed patients observed in our study remains unclear. However, it seems possible that decline in melatonin levels is due to impairment in adrenergic function that occurs in renal failure. Because the studies on the melatonin secretion in chronic renal failure bring about conflicting results, the relationship between renal diseases and melatonin secretion needs further investigations.