1. In a group of 11 normal individuals we measured glomerular filtration rate (GFR) by inulin clearances and effective renal plasma flow (ERPF) by p-aminohippurate clearances during a period of 24 h and a regimen of bedrest, identical food intake per 3 h and normal sleep/wake and light/dark cycles.

2. All subjects had a circadian rhythm for GFR with a maximum of 122 ml/min (sd 22) in the daytime, a minimum of 86 ml/min (sd 12) at night and with a relative amplitude of 33% (sd 15).

3. ERPF had a circadian rhythm with a similar relative amplitude as the GFR rhythm, but with a different phase. Because of this difference in phase, the calculated filtration fraction (GFR/ERPF) followed a circadian rhythm as well.

4. The circadian rhythms of urine volume and sodium excretion were in phase with the GFR rhythm, but the potassium rhythm had a different phase, probably because urinary potassium is largely derived from tubular secretion.

5. Urinary albumin and β2-microglobulin excretion had a circadian rhythm in phase with the GFR rhythm.

6. The highest quantity of sodium, water and β2-microglobulin was reabsorbed in the daytime; tubular reabsorption, expressed as percentage of the filtered load (fractional reabsorption), had a rhythm with a reversed phase.

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