1. Plasma levels of endothelin were measured in 30 patients with chronic renal failure, 32 patients on chronic haemodialysis treatment and 25 renal graft recipients with stable renal graft function. 2. In patients with chronic renal failure as well as in patients on regular haemodialysis treatment, mean plasma levels of endothelin were significantly increased (4.59 +/- 2.09 pg/ml, 10.08 +/- 3.12 pg/ml, respectively) when compared with normal subjects (1.88 +/- 0.6 pg/ml, P less than 0.01, P less than 0.001, respectively). 3. In the group with chronic renal failure a positive correlation between the plasma level of endothelin and the plasma concentration of creatinine was observed (P less than 0.003). 4. Renal graft recipients on cyclosporin A with stable renal graft function had a normal plasma level of endothelin suggesting that cyclosporin A nephrotoxicity is not mediated by endothelin. 5. Hypertensive patients with chronic renal failure or on regular haemodialysis and hypertensive renal graft recipients did not differ from the corresponding normotensive population with regard to the plasma level of endothelin, demonstrating that an increased plasma level of endothelin does not play a major role in the pathogenesis of renal hypertension.