The kidney is an important site of endothelin‐1 (ET‐1) production and is particularly susceptible to ET‐1 action. Infusion of ET‐1 in rats induces both functional and morphological alterations in the kidneys. Increased plasma level of ET‐1 has been reported in patients with chronic renal failure. However, there are still no reports on the plasma and urinary ET‐1 levels in patients with focal segmental glomerulosclerosis (FSGS). In the present study, we have measured the plasma concentration and urinary excretion rate of ET‐1 in 15 patients with nephrotic syndrome due to FSGS, and observed the serial changes of plasma and urinary ET‐1 in nephrotic rats with FSGS, induced by repeated injection with puromycin aminonucleoside (PAN). ET‐1 was measured with radioimmunoassay. The results showed that plasma ET‐1 concentration in FSGS patients was significantly higher than in normal controls (P < 0.05), and that urinary ET‐1 excretion rate was also significantly higher in FSGS patients than in normal controls (P < 0.01). In FSGS patients, the plasma and urinary ET‐1 was significantly correlated (P < 0.05), and the urinary ET‐1 excretion rate was significantly correlated with the amount of proteinuria (P < 0.05) and the glomerular sclerosing score (P < 0.01). In the ten rats with PAN‐induced FSGS, serial examination showed a significant increase in plasma ET‐1 after 8 weeks of injections, while the urinary ET‐1 excretion rate showed a biphasic increase that showed a peak after 4 to 6 weeks. The same changes in plasma and urinary ET‐1 levels were not observed in control rats injected with normal saline at the same frequency. Our results suggest that ET‐1 may be involved in the pathogenesis of FSGS in both humans and rats. J. Clin. Lab. Anal. 15:59–63, 2001. © 2001 Wiley‐Liss, Inc.