Circadian variations in proteinuria were studied in 17 patients with different types of glomerulopathies. During 3-4 successive days urine was collected over periods of 3 h under standardized conditions. Thirteen of the 17 patients showed a circadian rhythm of their proteinuria with a maximum excretion in daytime around 16.00 hours and a minimum excretion at night around 03.00 hours. In the majority of patients the urinary excretory rhythms of albumin, transferrin and immunoglobulin G were 'in phase' with each other and with the circadian rhythm of total protein excretion. Nine patients had a larger degree of rhythmicity for immunoglobulin G than for transferrin excretion. In eight of them a circadian rhythm of the selectivity index of proteinuria was seen with the lowest index at night. No relation was observed between the circadian rhythm of proteinuria and the type of glomerulopathy.