We analysed sodium excretion and its circadian variation in 70 patients with nephrotic syndrome and 19 healthy controls over 1-3 days, with a regimen of bed rest and constant sodium intake around the clock. We sampled urine and blood and took their blood pressure every 3 h. We also scored 60 renal biopsies for presence of interstitial fibrosis and tubular atrophy. Peripheral oedema was estimated in 37 patients. Fifty-nine patients excreted > 10 mmol sodium per 24 h, in equilibrium with dietary intake. In group A (n = 24), sodium excretion followed a normal circadian rhythm, with a daytime peak. In group B (n = 35), 29 had reversed circadian rhythm with a night-time peak, and 6 had no apparent rhythm. Nephrotic syndrome was more severe in group B than in A (serum albumin 19.5 vs. 24.1 g/l, p < 0.05; oedema 7.0 vs. 3.8 kg, p < 0.01). Group B also had signs of more advanced renal disease (GFR 49 vs. 99 ml/min; number of biopsies with tubulo-interstitial damage: 20/28 vs. 4/23; p < 0.001). Reversed sodium rhythm was associated with reversed circadian rhythms for GFR, effective renal plasma flow and urine flow, and blunting or reversal of the day-night differences in blood pressure and plasma renin activity. Eleven patients had urinary sodium excretion < 1 mmol/24 h. With respect to severity of nephrosis, they resembled group B, but GFR and incidence of tubulointerstitial lesions were like group A. Half of the patients with nephrotic syndrome had reversed circadian rhythm for sodium excretion.(ABSTRACT TRUNCATED AT 250 WORDS)