We have determined the plasma levels of endothelin-1 (ET-1) in patients with end-stage renal disease treated with continuous ambulatory peritoneal dialysis (CAPD). In 10 CAPD patients mean plasma ET levels (7.01 ± 0.5 pg/ml) were found higher than in normal subjects (3.6 ± 0.6 pg/ml). Peritoneal clearance of ET-1 was 1.58 ± 0.12ml/min and 2.3+0.2 ml/min during an isotonic peritoneal exchange and a hypertonic exchange, respectively. Plasma ET-1 levels decreased slightly but significantly during a hypertonic 4-hour exchange (from 7.01 ± 0.5 to 6.14 ± 0.4 pg/ml) (p < 0.05) and were not modified by an isotonic 4-hour exchange (from 5.7 ± 0.5 to 6.4 ± 0.4 pg/ml). ET-1 is known to be an important vascular stimulant and thus elevated ET-1 levels may play a role in the genesis of cardiovascular complications which are the leading cause of mortality in these patients.