The changes in urinary .beta.2microglobulin excretion during the day were studied in 20 patients with a nephrotic syndrome under standardized conditions and compared with the known circadian variations in urinary albumin excretion in the same patients. Fifteen of the 20 patients (75%) had a circadian rhythm for .beta.2microglobulin in the urine. Phase as well as relative amplitude of the .beta.2microglobulin circadian rhythm varied between patients. In addition, its presence, normality, amplitude and phase were largely different from the albumin rhythm. Observations made in 10 healthy individuals suggest that day-night variability in the excretion of .beta.2microglobulin and albumin in the urine is also present in the absence of clinically important proteinuria. Circadian variations in urinary .beta.2microglobulin excretion are found in a majority of the patients with nephrotic syndrome and as they differ in several respects from the day-night fluctuation in urinary albumin excretion the mechanism is probably different.