The melatonin status of patients in end-stage chronic renal failure (CRF) was evaluated by the determination of daytime plasma melatonin levels and by the investigation of the circadian rhythmicity of melatonin secretion. A significant increase in plasma melatonin concentration was found in all CRF patient groups investigated, i.e. CRF patients on conservative treatment (CT; n = 48), CRF patients on maintenance haemodialysis treatment (HD; n = 39) and CRF patients on peritoneal dialysis (PD; n = 32). Successful transplantation led to a marked reduction in plasma melatonin levels. The circadian rhythm of melatonin secretion would appear to be suppressed in CRF as the nocturnal secretory surge was absent in all HD patients and in 80% of the posttransplantation patients studied.